TomG:
I found the exception was generated by the lack of a UserTransaction being included in the "env" object passed to JPAKnowledgeService.newStatefulKnowledgeSession(). Once I did that, all appears to be working.
I have the same problem, when I configure process and task persistence together.
Here is an exmaple http://community.jboss.org/people/bpmn2user/blog/2011/03/25/jbpm5--persistence-hello-process-example
Could you share the information about including the 'UserTransaction' in env?