Nathan Dunlap, WPF designer extraordinaire, put together a fantastic screencast and code sample that shows how to create workflow between Expression Interactive Designer and Visual Studio.  Nifty stuff like creating a .dll in VS with dependency properties that then can be "drawn" in EID as well as the custom properties showing up in the properties pane of EID. 

I also interview him on Channel 9 where he discusses his experience as a designer on WPF applications, everything from dozens of the SDK samples to his work on the  NY Times app.  It isn't on the homepage yet, but should be soon!