[UPDATE: The Xaml Exporter now is hosted on CodePlex (here)]
I checked in an updated Blender exporter to 3D Tools for Windows Presentation Foundation which adds support for double sided materials and flat shaded geometry. I have not yet package a new release, so for the moment you’ll need to grab xaml_export.py from source control.
These features are accessible via the circled buttons on the material and mesh panels during editing. Thanks to SEISENBERG for reporting this in the message board.