Acrylic
Having just tried "Acrylic", its quite clear its Microsoft's answer to some kind of low-end vector art program.
Its bitmap capbilities leave a lot to be desired, and the UI seems unprofessional and unfinished (I particularly dislike the bevel/shadow effect under the controls) and rather clunky, much like the edges of Adobe Palletes and parts of Corel Painter, and to an extent, Macromedia's MX 2004 interface (the worst ever, imho)
Check this out :
http://www.microsoft.com/products/expression/
I'm guessing either Acrylic will have a Developer Edition for creating XAML stuff, or portions of Acrylic will be ported into a Visual Studio 2005 designer. As I write this, I have about 20 minutes left in the download of Acrylic Beta. Let's see what it holds.