Can't really answer that in a general way.
However I had exactly the same problem and updating Hibernate fixed it. So far I can't see any new problems...
I use Hibernate 3.2.6 since it was released without problems, too. I recommend to upgrade. But you should consider that seam is totally independent of the specific version. You must put in the AS-lib directory to get it work. Therfore you might get problems with other AS components. Maybe ask this question in the AS forum.