This is a bit of a self-serving post but probably of interest to many IXDA folks.
Today the first public version of Expression Blend 3.0 was released. Included is a new feature called SketchFlow which is designed for dynamic-prototyping.
If you're a fan of Bill Buxton and his ideas put forth on Sketching for User Experience in his book and past IxDA talks SketchFlow is worth checking out. It's got fantastic features to allow designers to bring sketches to life and build complex prototypes in an rapid and iterative process without having to resort to code.