How to build your optical "mouse"?

Use a surveillence camera. Same principle as optical mouse, just use a black & white camera.

Much easier for a tiny digital camera to 'see' different brightnesses of red (as in optical mouse) than to differentiate between colours and brightnesses, etc. So use a black and white cam, nice bright red or infrared lighting for it to "see" better, stick a filter over the camera and it's even easier, as it's only picking up different shades, not diff colours & brightness/saturation/whatever for each colour. Just red, shades from black to bright.

-mark