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.