See this discussion thread: http://groups.google.com/group/liftweb/browse_thread/thread/35245dcd781c4d1b

Symptom:
I found that when creating mapped objects and saving them within a loop, the object references are retained indefinitely and are thus not garbage collected, leading eventually to out of memory errors.
This only happens if S.addAround(DB.buildLoanWrapper()) is used.

This was raised in the Google discussion group, and in David Pollak's words:
"If there are no post-commit functions defined for the MetaMapper,
we won't track Mapper items for the non-existent post-commit functions. "