Finally: Host your WF Workfows in Biztalk Server 2006!
Now you can do this with the just released BizTalk Server 2006 extensions for Windows Workflow Foundation SDK.
You can download it here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=b701c00f-cdc1-4edb-a975-b9412263ec6e&displaylang=en
And check the comments of Paul Andrew here:
http://blogs.msdn.com/pandrew/archive/2007/11/01/just-released-biztalk-server-2006-extensions-for-windows-workflow-foundation-sdk.aspx
Enjoy!