I just came across this project by the Columbia University Computer Graphics & User Interfaces Lab which I think is definitely worth looking at.
In their own words, Goblin XNA is a platform for research on 3D user interfaces, including mobile augmented reality and virtual reality, with an emphasis on games. It is written in C# and based on the Microsoft XNA platform.
One very cool project that have already been implemented with the platform are is the Goblin XNA AR Racing Game
The project has been supported by Microsoft Research and its source code can be downloaded from CodePlex.
This is a very interesting use of a technology that has been created to create games and is being used to do some very serious research.