I have being doing some research on a mixed reality application using 
ARToolkit, and have been experimenting with the use of 3D fiducials.

I would be interested to hear from anyone else that have tried this sort 
of thing.

There are some details of an early prototype here:

http://spacecube.jeremygoslin.com



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Jeremy Goslin
Geneva Interaction Lab, Université de Genève.
http://www.jeremygoslin.com