The BizTalk Server Developer Center has been updated to highlight integration with WF and WCF.
http://msdn.microsoft.com/biztalk/

Windows Workflow and BizTalk Server 2006 Code Sample
This sample demonstrates how to integrate Microsoft BizTalk Server 2006 and Windows Workflow Foundation through Web services. 

Reference Application Pack for Retail
This white paper, along with the sample application, showcases integration using service orientation with BizTalk Server 2006, Windows Communication Foundation, and Microsoft Business Scorecard Manager.

Windows Workflow Foundation Overview
Windows Workflow Foundation is the programming model, engine, and tools for quickly building workflow-enabled applications on Windows. It consists of a namespace, an in-process workflow engine, and designers for Visual Studio 2005.