The Expression Graphic Designer (code-named Acrylic) is capable of saving out assets to Xaml using the File > Export > Xaml Export from the main menu. The assets can be exported as elements in a canvas, or as a resource (suitable for embedding in your application).
Unfortunately unlike the Expression Interactive Designer (code-named Sparkle) Xaml is not the "native format" of Acrylic, so the export process is one-way. The representation in Xaml is quite good however, even for assets that contain both vector and raster (bitmap) layers.
The March CTP of the Expression Graphic Designer can be downloaded from here.