This week's book giveaway is in the Other Languages forum. We're giving away four copies of Functional Reactive Programming and have Stephen Blackheath and Anthony Jones on-line! See this thread for details.
Hibernate is considered to be a lot more lightweight because you can use it to persist POJOs, whereas EJBs (specifically Entity if we are talking persistence) are a lot more heavyweight. This however is excpected to change with the release of EJB3.
Have a look through the posts in the ORM forum and you will see all these points discussed.