Uncable to commit: transaction marked for rollback

I'm intermittantly getting this error message in a java app.
using Geronimo / Hibernate / Postgres 8.3.9

javax.transaction.RollbackException: Unable to commit: transaction marked for
rollback

Can someone give me a scenario where this would happen? "unable to commit"
makes everyone immediatly go to database issue. But it seems like an app
issue to me.

I was thinking that maybe it's a 2 phase commit / XA or something like that.
(TX open, phase 1 commit fails, phase 2 commit throws this error?)

I can't imagine how this would happen within a single transaction setup.

Thanks!

Dave