Using the built-in motion sensors is a bit of a problem, because only a small fraction of computers have them, and there isn't a standardized interface for them. It already bothers a lot of kids that there are mysterious blocks on the "sensing" panel that only work if you have a Scratchboard. Increasing the hardware-specific features of scratch would probably be a bad idea.
Web cams are a bit more common, but analyzing video input is fairly complicated.
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