Sylvain Duford has a post where he describes K2.Net and his reaction to a demo of their recent stuff.  I agree with Sylvain's assumptions.  I've been watching K2.Net for a while now, and I am also very impressed with their stuff.  Thay are a very strong complement to BizTalk workflow. 

There are other workflow providers who are good partners to Microsoft, such as Captaris-Teamplate.  And for some projects, the Human Workflow Services provided within BizTalk is sufficient.  Perhaps it is best to consider all three alternatives when determining the best fit for your implementation.

If you have experience with workflow and BizTalk and are looking for someone to talk to about it, send me a note or post a comment here...