Distribution of the small mouse cameras would be neat, and some small breakout board would be a killer, the optics would be out of this world hehe =D, most of this devices have both quadrature outputs and I2C-like interface.
I could be pointless if you consider an optical mouse is around $6 but everytime I buy one of those I need to get the camera out of the mouse and make my own pcb, that ends up being just a couple of mm bigger than the chip itself witch is only about a square cm.