Okay this is interesting. It seems that we've worked out a way to build workflow models inside of Windows Workflow Foundation (WF) and host them in BizTalk Server.
From the post on Paul Andrew's blog:
The BizTalk Adapter for Windows Workflow Foundation SDK Sample (June 2007 CTP) is a preview of some new technology for business logic and process developers. It provides for workflow models developed using Windows Workflow Foundation from the .NET Framework 3.0 to be hosted in BizTalk Server 2006. By doing this the workflow gains access to services from BizTalk Server 2006 including scalability, reliability, manageability, and access to messaging with BizTalk Server ports.