Compilation of links to blogs/resources on comparison between BizTalk and WF:
BizTalk Server 2004/2006 and Windows Workflow Foundation (Brian Loesgen)
What about BizTalk Indeed? (Scott Woodgate)
Windows Workflow Foundation vs BizTalk Server (Lamont Harrington)
Windows Workflow Foundation - Where does BizTalk fit in? (Darren Jefford)
BizTalk vs. Windows Workflow Foundation (WF) (Chunyu)
Understanding BizTalk Server 2006 (MSDN)
Integrating BizTalk Server 2006 and Windows Workflow Foundation (Level 200) (MSDN WebCast)