Hibernate will be useful in design phase
Hibernate is an impementation, not a design tool. I don't know if it really brings anything to the design phase. If you design your software using OO concepts you are basically writing in the requirement to have an ORM layer somewhere - either writing it yourself or using some third party tool such as Hibernate. And its presence should not (in my opinion) influence any ER modelling decisions, despite Hibernate's docs suggesting you can design data models from the Object model down.
(BTW: this wasn't the question asked but you mention it in your reply: 5000
entities would probably strongly suggest some sort of mapping layer, since an entity = one
table not one record. So you would be looking at a fairly big ER model)
[ October 07, 2005: Message edited by: Paul Sturrock ]