I had a single file open, opening another showed a tab with the file name. However tabbing between files wouldn't change the editor contents.
I closed both tabs and tried opening a different file.

My system is:
OS X 10.6.6

java version "1.6.0_24"
Java(TM) SE Runtime Environment (build 1.6.0_24-b07-334-10M3326)
Java HotSpot(TM) 64-Bit Server VM (build 19.1-b02-334, mixed mode)

eclipse Build id: M20100211-1343

Dummy SDT feature for upgrade 2.8.0.scala-ide-upgrade ch.epfl.lamp.sdt.feature.group
Dummy weaving feature for upgrade 1.6.4.scala-ide-upgrade org.eclipse.contribution.weaving.feature.group
Eclipse SDK 3.5.2.M20100211-1343 org.eclipse.sdk.ide
JDT Weaving for Scala 1.0.0.201103210007 org.scala-ide.sdt.weaving.feature.feature.group
Scala IDE for Eclipse 1.0.0.201103210007 org.scala-ide.sdt.feature.feature.group
Scala IDE for Eclipse Source 1.0.0.201103210007 org.scala-ide.sdt.source.feature.feature.group

eclipse.ini

-startup
../../../plugins/org.eclipse.equinox.launcher_1.0.201.R35x_v20090715.jar
--launcher.library
../../../plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.0.1.R35x_v20090707
-showsplash
org.eclipse.platform
--launcher.XXMaxPermSize
256m
-vmargs
-Xms40m
-Xmx2048m
-Xdock:icon=../Resources/Eclipse.icns
-XstartOnFirstThread
-Dorg.eclipse.swt.internal.carbon.smallFonts

!ENTRY org.eclipse.ui 4 0 2011-03-23 10:12:25.875
!MESSAGE Unhandled event loop exception
!STACK 0
scala.tools.nsc.symtab.Types$TypeError: illegal cyclic reference involving type List
	at scala.tools.nsc.symtab.Types$class.typeRef(Types.scala:2576)
	at scala.tools.nsc.symtab.SymbolTable.typeRef(SymbolTable.scala:13)
	at scala.tools.nsc.symtab.Types$TypeMap.mapOver(Types.scala:2932)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:3313)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:3189)
	at scala.tools.nsc.symtab.Types$TypeMap.mapOver(Types.scala:2953)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:3313)
	at scala.tools.nsc.symtab.Types$Type.asSeenFrom(Types.scala:516)
	at scala.tools.nsc.symtab.Types$Type.computeMemberType(Types.scala:540)
	at scala.tools.nsc.symtab.Symbols$MethodSymbol.typeAsMemberOf(Symbols.scala:1780)
	at scala.tools.nsc.symtab.Types$Type.memberType(Types.scala:531)
	at scala.tools.nsc.symtab.Types$Type.findMember(Types.scala:874)
	at scala.tools.nsc.symtab.Types$Type.nonPrivateMembers(Types.scala:462)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser$Owner$class.addForwarders$1(ScalaStructureBuilder.scala:102)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser$Owner$$anonfun$complete$1.apply(ScalaStructureBuilder.scala:190)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser$Owner$$anonfun$complete$1.apply(ScalaStructureBuilder.scala:163)
	at scala.collection.mutable.HashMap$$anonfun$foreach$1.apply(HashMap.scala:80)
	at scala.collection.mutable.HashMap$$anonfun$foreach$1.apply(HashMap.scala:80)
	at scala.collection.Iterator$class.foreach(Iterator.scala:631)
	at scala.collection.mutable.HashTable$$anon$1.foreach(HashTable.scala:161)
	at scala.collection.mutable.HashTable$class.foreachEntry(HashTable.scala:194)
	at scala.collection.mutable.HashMap.foreachEntry(HashMap.scala:39)
	at scala.collection.mutable.HashMap.foreach(HashMap.scala:80)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser$Owner$class.complete(ScalaStructureBuilder.scala:163)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser$Builder.complete(ScalaStructureBuilder.scala:727)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser.atBuilder(ScalaStructureBuilder.scala:837)
	at scala.tools.eclipse.javaelements.ScalaStructureBuilder$StructureBuilderTraverser.traverse(ScalaStructureBuilder.scala:763)
	at scala.tools.eclipse.javaelements.ScalaCompilationUnit$$anonfun$buildStructure$1.apply(ScalaCompilationUnit.scala:96)
	at scala.tools.eclipse.javaelements.ScalaCompilationUnit$$anonfun$buildStructure$1.apply(ScalaCompilationUnit.scala:87)
	at scala.tools.eclipse.util.Cached$$anonfun$apply$1.apply$mcV$sp(Cached.scala:22)
	at scala.tools.eclipse.util.Cached$$anonfun$apply$1.apply(Cached.scala:21)
	at scala.tools.eclipse.util.Cached$$anonfun$apply$1.apply(Cached.scala:21)
	at scala.tools.eclipse.util.Cached$class.locked(Cached.scala:83)
	at scala.tools.eclipse.util.Cached$class.apply(Cached.scala:20)
	at scala.tools.eclipse.ScalaPresentationCompiler$CachedCompilerResult.apply(ScalaPresentationCompiler.scala:64)
	at scala.tools.eclipse.ScalaPresentationCompiler.withCompilerResult(ScalaPresentationCompiler.scala:99)
	at scala.tools.eclipse.ScalaProject$$anonfun$withCompilerResult$1.apply(ScalaProject.scala:367)
	at scala.tools.eclipse.ScalaProject$$anonfun$withCompilerResult$1.apply(ScalaProject.scala:366)
	at scala.tools.eclipse.util.Cached$$anonfun$apply$1.apply$mcV$sp(Cached.scala:22)
	at scala.tools.eclipse.util.Cached$$anonfun$apply$1.apply(Cached.scala:21)
	at scala.tools.eclipse.util.Cached$$anonfun$apply$1.apply(Cached.scala:21)
	at scala.tools.eclipse.util.Cached$class.locked(Cached.scala:83)
	at scala.tools.eclipse.util.Cached$class.apply(Cached.scala:20)
	at scala.tools.eclipse.ScalaProject$$anon$1.apply(ScalaProject.scala:40)
	at scala.tools.eclipse.ScalaProject.withPresentationCompiler(ScalaProject.scala:362)
	at scala.tools.eclipse.ScalaProject.withCompilerResult(ScalaProject.scala:366)
	at scala.tools.eclipse.javaelements.ScalaCompilationUnit$class.withCompilerResult(ScalaCompilationUnit.scala:58)
	at scala.tools.eclipse.javaelements.ScalaSourceFile.withCompilerResult(ScalaSourceFile.scala:40)
	at scala.tools.eclipse.javaelements.ScalaCompilationUnit$class.buildStructure(ScalaCompilationUnit.scala:87)
	at scala.tools.eclipse.javaelements.ScalaSourceFile.buildStructure(ScalaSourceFile.scala:40)
	at org.eclipse.jdt.internal.core.Openable.generateInfos(Openable.java:258)
	at org.eclipse.jdt.internal.core.JavaElement.openWhenClosed(JavaElement.java:515)
	at org.eclipse.jdt.internal.core.JavaElement.getElementInfo(JavaElement.java:252)
	at org.eclipse.jdt.internal.core.JavaElement.getElementInfo(JavaElement.java:238)
	at org.eclipse.jdt.internal.core.CompilationUnit.getSourceRange(CompilationUnit.java:908)
	at org.eclipse.jdt.internal.ui.javaeditor.selectionactions.GoToNextPreviousMemberAction.update(GoToNextPreviousMemberAction.java:90)
	at org.eclipse.jdt.internal.ui.javaeditor.selectionactions.GoToNextPreviousMemberAction.<init>(GoToNextPreviousMemberAction.java:69)
	at org.eclipse.jdt.internal.ui.javaeditor.selectionactions.GoToNextPreviousMemberAction.newGoToNextMemberAction(GoToNextPreviousMemberAction.java:56)
	at org.eclipse.jdt.internal.ui.javaeditor.JavaEditor.createActions(JavaEditor.java:2718)
	at org.eclipse.jdt.internal.ui.javaeditor.CompilationUnitEditor.createActions(CompilationUnitEditor.java:1037)
	at scala.tools.eclipse.ScalaSourceFileEditor.createActions(ScalaSourceFileEditor.scala:34)
	at org.eclipse.ui.texteditor.AbstractTextEditor.createPartControl(AbstractTextEditor.java:3465)
	at org.eclipse.ui.texteditor.StatusTextEditor.createPartControl(StatusTextEditor.java:53)
	at org.eclipse.ui.texteditor.AbstractDecoratedTextEditor.createPartControl(AbstractDecoratedTextEditor.java:427)
	at org.eclipse.jdt.internal.ui.javaeditor.JavaEditor.createPartControl(JavaEditor.java:3078)
	at org.eclipse.jdt.internal.ui.javaeditor.CompilationUnitEditor.createPartControl(CompilationUnitEditor.java:1475)
	at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:662)
	at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:462)
	at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:595)
	at org.eclipse.ui.internal.PartPane.requestActivation(PartPane.java:263)
	at org.eclipse.ui.internal.EditorPane.requestActivation(EditorPane.java:98)
	at org.eclipse.ui.internal.presentations.PresentablePart.setFocus(PresentablePart.java:192)
	at org.eclipse.ui.internal.presentations.util.TabbedStackPresentation$1.handleEvent(TabbedStackPresentation.java:94)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.fireEvent(AbstractTabFolder.java:270)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.fireEvent(AbstractTabFolder.java:275)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.handleMouseDown(AbstractTabFolder.java:345)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder$3.mouseDown(AbstractTabFolder.java:79)
	at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:179)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
	at org.eclipse.swt.widgets.Display.sendEvent(Display.java:3543)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1250)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1273)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1258)
	at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:1079)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3441)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3100)
	at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2405)
	at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2369)
	at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2221)
	at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:500)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:493)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
	at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:113)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:194)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:368)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:559)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1311)

!ENTRY org.eclipse.ui 4 0 2011-03-23 10:12:26.919
!MESSAGE Unhandled event loop exception
!STACK 0
java.lang.NullPointerException
	at org.eclipse.ui.internal.presentations.PresentablePart.setFocus(PresentablePart.java:188)
	at org.eclipse.ui.internal.presentations.util.TabbedStackPresentation$1.handleEvent(TabbedStackPresentation.java:94)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.fireEvent(AbstractTabFolder.java:270)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.fireEvent(AbstractTabFolder.java:275)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.handleMouseDown(AbstractTabFolder.java:345)
	at org.eclipse.ui.internal.presentations.util.AbstractTabFolder$3.mouseDown(AbstractTabFolder.java:79)
	at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:179)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
	at org.eclipse.swt.widgets.Display.sendEvent(Display.java:3543)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1250)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1273)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1258)
	at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:1079)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3441)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3100)
	at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2405)
	at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2369)
	at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2221)
	at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:500)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:493)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
	at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:113)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:194)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:368)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:559)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1311)