PC Ball Mouse = Rotary Position Sensor
Computer mice with balls contain two (or sometimes three, if the have a scroll wheel) rotary position sensors. These are not mechanical switches, but optical ones. (Just to confuse you, this is why "ball mice" are "optical mice".) A pair of receivers is placed on the opposite side of a slotted wheel from a transmitter. As the ball rotates, it spins the wheel and the sensors detect the direction and speed. Two such arrangements are placed perpendicual to one another.
If anyone wants to try (I'd love to do this, if I could find the time) PS/2 or serial port mice have a little chip in them which does all the speed/position decoding for you AND outputs a serial data stream containing the information. I know this because in a past life I hooked a serial mouse to a PIC with sucess.
If anyone has the time, and an old mouse, please take it apart, attach the slotted wheels to your robot's wheels, hook the chip to your PICaxe and decode the data stream.
The data stream is documented here.
These would make an ideal (and virtually free) position sensor.
Tell us how you get on.