Walt is right, it should be available in a "FUTURE" preview.. The current mix 09 preview release is just targeting Silverlight 3...
Don Burnett
Just as a slight clarification, the MIX 09 Preview Release works quite well for WPF too and all of the new features (Behaviors/Effects/etc) work for WPF. Hopefully we will get SketchFlow soon though because it would really help our spec process.
Brent