Just announced at TechEd 2007 (US) - Host your Windows Workflow in BizTalk Server via the BizTalk Adapter for Windows Workflow Foundation sample (June 2007 CTP).  For complete details see Paul Andrew's blog for the complete details (http://blogs.msdn.com/pandrew/archive/2007/06/04/biztalk-adapter-for-windows-workflow-foundation-sample-june-2007-ctp.aspx).

There is a video demo of this technology by Jon Flanders here.