A quick update on selecting different persistence providers.
I gave openjpa a quick spin last weekend (as a top level deployment), using their "all" jar (nice having everything needed in one jar). It was an interesting first attempt that got these errors when creating the "container entity manager factory" http://pastie.org/1777575. I started a branch for this effort that I will move more of my experiental code to soon. If anyone wants to hack on these types of problems, the branch is https://github.com/scottmarlow/jboss-as/tree/multipleproviders_2.
Also in the above branch, is some experimental code to do some potentially evil bridging of the AS7 JPA container/integration layer and a persistence provider that is bundled with a deployment (e.g. in the ear lib). One of the challenges is coming up with integration classes (for bridging the persistence provider with AS7) that don't pull in jars that could conflict with the application deployment. http://pastebin.com/dCvxvdG1 is a CCE caused by class org.jboss.as.jpa.hibernate.HibernateAnnotationScanner being loaded by a different classloader than the target class org.hibernate.ejb.packaging.Scanner (I was trying to test a bug fix I had just made in Hibernate 4.0). For this to work, I think we need enough integration code to accompany the persistence provider (rather than relying on AS7 jars directly).
Applications should be fine with packaging a persistence provider and using it natively (one of the advantages of the AS7 modular classloading).
If you want to help out in other areas, testing is pretty important. Already, I have had a few people reporting issues based on their testing efforts.
If you want to contribute to other areas of JPA (or AS7), find me on On IRC - irc://irc.freenode.org/jboss-as7 or https://webchat.freenode.net/?channels=jboss-as7.