What's preventing you from using AS' bundled Hibernate?
The simplest of answers is time.
Hibernate 3.6 would require an update of Spring (based upon the AbstractMethodError in my earlier post) which would require an update of Acegi Security (to Spring Security) and so on. While those upgrades are inevitable (and time consuming), we had hoped that we would be able to upgrade JBoss AS without having to upgrade our application at this time (or the 3rd party libraries that it depends upon).