Step 1: The Idea
If a shadow is detected over the central and bottom center photocell then the mouse will be stationary. A shadow over every cell in the middle vertical column indicates a up movement. Just the bottom center is a down movement. Right center and right bottom is right, all the right hand row is up/right, just bottom right corner is right/down, and all this mirrored for left movement.
It's a fairly simple design, and the device itself wouldn't be hard to create. The challenge would be in the programming, and even that shouldn't be too overly difficult. If you got really creative you could very the speed of the pointer by watching how much shadow is over a particular photocell. If you used an IR led and a photocell capable of detecting IR light you could make a design that won't light up the room and create glare, while not being to influenced by bright lighting.