Charles Sterling has put together a great 3 part Hands on Lab where you start off using Visual Studio 2008 and the new WPF Designer (code name "Cider") to build an Browser Application (.xbap).  Then, he walks you through how a designer could pick up the app you built in Visual Studio and jazz it up using Expression Blend.  Going through these three HOLs is a great way to try out the designer/developer workflow we've been saying that Expression/Visual Studio gives you and learn what you will be able to do as a developer using the WPF Designer in Visual Studio 2008.  Labs are here.

-Marc