I installed a bicycle odometer on my bike a little while ago, and must say, I'm very happy with the results.  It was one of the cheapo's from the on base exchange, with mph and mileage both on the same screen.  I mounted the readout on my handlebars, so I can look down and see it a lot easier.  Ran the cable down my fork, leaving enough slack to allow for the suspension to flex, without having so much that the cable could get ripped off.  the tricky part was attaching the sensors to the fork and spokes, but I got the sensor to attach to the spoke, and then used some Amazing Goop to hold it in place, and then attached the other sensor to the front fork using zipties.  It was too far away to make a reading at first, so I had to find a way to push it closer to the forks, so it would actually register the magnet, but since I was using spare parts anyways, I took an old cb clamp, and ziptied that to the forks, and then gooped the sensor to that...it works great, calibrated fine, it's right on the money, as far as the tire size goes.