WF 4.0 Introduction
http://channel9.msdn.com/learn/courses/VS2010/WCFWF/IntroToWF/
WF 4.0 Blog
http://blogs.msdn.com/b/mwinkle/
Mapping WF 3 activities to WF 4
http://www.pluralsight-training.net/community/blogs/matt/archive/2009/06/26/mapping-wf-3-activities-to-wf-4.aspx
Workflow Foundation Internals
http://blogs.msdn.com/b/flow/
WF 4 Migration Guidance
http://www.microsoft.com/downloads/details.aspx?FamilyID=bd94c260-b5e0-4d12-93ec-53567505e685&displaylang=en#filelist