Michael, using Activiti (or the PVM in Activiti) makes perfectly sense, which we are currently discussing with the Spring guys. If you remember, jBPM was also used in Seam for PageFlow (looks like WebFlow). So its certainly something on our agenda/wishlist.
Hum, so a this stage we would be looking at having to make a choice between the two? If so it would be good if you could may be blog about this when you and the "spring guys" have got some form of answer/roadmap for the two.
adding to that: BPMN is concrete on our roadmap and that will be ready (GA) by November. Whereas spring webflow would be switched is still undecided. Certainly possible, but my current estimation is that this will be rather unlikely as it would be a lot of work, that doesn't result into new features. Just a different library dependency for spring webflow. But spring people might have different thoughts on that.