Our WF evangelist, Ron Jacobs, just posted a 10-4 screencast that walks through WF4 as it's looking in VS 2010.  This is a good first look at our bits since PDC (we've changed things a little bit).  I really like that he takes it from a unit testing perspective.  There are certainly things that have been added (like WorkflowInvoker.Invoke) which will make testing activities a lot easier than the 3.0 days.

Check it out, let me know if you've got feedback. 

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