MacStories points to an interesting use of the iPad's front facing camera to provide a convincing 3D effect on the iPad. Jeremie Francone and Laurence Nigay from the Laboratory of Informatics of Grenoble put together this demo video showing how user tracking can be used:
We track the head of the user with the front facing camera in order to create a glasses-free monocular 3D display. Such spatially-aware mobile display enables to improve the possibilities of interaction. It does not use the accelerometers and relies only on the front camera.