Rene Schulte has done some work integrating the NyARToolkitCS with Microsoft’s Silverlight 4 platform and published a neat little Augmented Reality proof-of-content tool on his website.
I use the open source Augmented Reality library NyARToolkitCS and implemented the necessary interfaces to make it work with Silverlight. The NyARToolkitCS library is a completely managed version of the well known ARToolkit.
You can find the printable marker and application on his website. Requires Silverlight 4, obviously.