This is my current persistence.xml, I believe I added the <jta-data-source> stuff after the original post.
Here it is:
In my pom.xml I do *not* have a reference to jbpm-test as it causes a conflict with "org.jbpm.persistence.jpa"
I also manually changed (throught the JBoss console) the "H2DS" datasource installed by default with the jbpm-installer to point to my local (postgres) database instead of the H2 one. (Since the have the same persistence-unit name).
Let me know if you need more info.