I've been a fan of this idea for a long time (think of those miniscule portable mice
!) One of them would be great for detecting slippage, I imagine. A while back I went so far as googling usb-I2C converters, and they do exist, for a price. I'm sure other people on this forum know more about the feasibility of converting usb and I2C. If that works, you might have a bit of work at getting mouse drivers to work properly on an NXT as well.