I'm just learning Hibernate myself at work, and it ROCKS.
Cool, I'll implement one regardless of if needed or not.
I think right now all you'll need is one hibernate bean. Where hibernate really shines is the relationships. I'm just amazed.
there is just one table at the moment, but there might be more later. Just doing this to learn it really, and if it is useful for somebody else all the better.
The point of making it pluggable is that someone will implement others :-). Non-JMS delivery, other mailboxes, etc.