Perhaps not even up, the cameras could follow your movements on the surface of a desk as well... I don't think you'd want it to replace your keyboard and mouse, but I could see a few instances where quick gestures would work nicely. widgets, games, maps, selecting images, manipulation of 3-d elements. The key to making it work would lie in being sensitive enough to not require overt motions, but smart enough to discard unintended ones. If I have to lift my arms above the mac for everything, it's not going to be of much use.
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