common problem with Hibernate/NHibernate and child IDs

Question

I'm asking both Hibernate and NHibernate groups because I'm thinking this may be a common issue seen on both.

What does it usually mean when you call a saveOrUpdate on a child object and...
1) If it is an insert everything works fine.
2) If it is an update its wiping out the parent ID in the database.

More info: problem is really occuring on Java