The Hibernate version included in JBoss 4.0.3 isn't usable. As soon the session of a har deployed Hibernate-Factory calls Session.beginTransaction, a naming exception (UserTransaction not bound) is thrown.

Having a look in JNDIView i see that UserTransaction is bound to global namespace.

As a work around I replaced the Hibernate Jars (hibernate3, hibernate-persistence) with the ones from JBoss 4.0.3RC2. Everything works fine now.