I bought some of the $.69 mice, and I haven't been able to hack it so far. Like Dirt says, the infrared sensor has 4 legs, so I'm not sure what I'm supposed to do with that. I played with the multimeter and made some uneducated guesses that the legs would be something like x1,x2,+5v,and G. When I connected it to the trackball with all 4 wires going to the trackball, I was able to get a slight reaction out of the thing. If I tapped the trackball in one direction, the mouse pointer would move 1 pixel, and vice versa for the other direction. However, if I really gave the thing a spin, I would see no reaction from the mouse pointer.
I'm going to post in the main forum to see if anyone knows anything about these mysterious 4 legged sensors.