Tom Baeyens has written a nice blog introduction to a technical article on the Process Virtual Machine.

Tom's blog is great, because it gives you an understanding of the problems and commonalities between various process/workflow/orchestration technologies. I faffed about with Werkflow though a few revisions, all failing miserably.

I think workflow (and rules) will ultimately be important programming models in the future. While developers may find it hard to transition from imperative to declarative thinking, both of these technologies map more closely to how Real Humans tend to think about problems. This is evidenced by the "flow charts" that match the shape of a business process. We're better at drawing pictures than describing with words the steps involved in taking a thing through a process.