I keep meaning to blog about this - better late than never. One challenge for all of us working with WinFX is the lack of tools for developing presentation with - the January CTP of WinFX gave us a better drop of Cider (The feedback I got on the Dec CTP drop was not great) and now we also have a CTP of Sparkle.
Download Sparkle from http://www.microsoft.com/products/expression/en/interactive_designer/id_free_trial.aspx (install order at the bottom of the page).
NB: In the current builds the cider designer is only available on the full version of Visual Studio (Professional and above). If you install it on Visual C# Express or Visual Basic Express, you'll only get the IntelliSense and templates, rather than the tabbed design view.
For an explanation of the difference between Cider and Sparkle check out http://blogs.msdn.com/ericnel/archive/2005/12/13/503175.aspx