Yay, I get to give out information that took me days to discover: behold, OpenCV
Open source computer vision library. Requires a TON of processing so microcontrollers aren't going to be fast enough unless you have a whole bunch (probably hundreds). Even my Eee PC was really choppy, though a good portion of that was probably the overhead of the operating system and window manager.
If you want to play around with it and you have a linux machine you can check out harpia. It uses block objects to visually program what you want it to do. Not really that intuitive but much easier than starting with the library from scratch. I don't know what the website for it is, but under Ubuntu: