Updated Sept 12 2008: moved from Devel site, added new links.
I must admit that I have been skeptical about the utility of full-featured, full-blown business process languages, especially for use in what I anticipate to be a simplified interface for the Semantic Web. If the language isn't fairly intuitive, people just aren't going to understand it.
Workflow modeling can be a very technically-oriented subject, often requiring that the practitioner understanding a specialized notation. Most of this content is on the Development site ( link above ).