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I built a similar device, although mine measures linear speed instead of RPM.
I used the same mouse sensor idea, except I used cut down the base of the mouse to keep the whole axle assembly and the ball contact part of the sensor. I filled the slots and drilled a new hole then mounted IR sensor/receiver pair. The output runs through a small transistor circuit to digitize the output pulse.
Rather than go to the trouble of building a real interface for it (microcontroller and LCD display), I just connect it to the 'scope and to determine the frequency, then multiply by the diameter of the input shaft.
Works well to determine the top speed of the car, although its not very self-contained, since I have to keep the 'scope around to use it.
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