@aL_ : I agree. As a developer of both silverlight and WPF applications, it would make it a lot easier to share code and resources between the two.
Also, I am interested to see the details around the 3D api. I'm hoping it's a low enough level API that the XNA team could conceivably support it as a surface for XNA games with good (enough) performance. This is one way they could allow XNA/Silverlight integration in future versions of Windows Phone, assuming SL5 is ported to that platform.