Yes, they can, and some cyclocomputers are made to be mounted that way. I bought my vintage Trek 520 from a guy here in town, and it came with a Cateye Astrale (also a vintage piece) already installed; it has a magnet on the left crank with a sensor on the front part of the chainstay for measuring cadence, and a magnet on the inner rear wheel with sensor on the rear part of the chainstay for measuring speed. It, however, was made to be hooked up this way. Other cyclocomputers can be Jerry-rigged to similarly work on the rear wheel. I took a $10 Bell (-gasp- Wal-Mart brand!) cyclocomputer and routed the cable up over the toptube and down the chainstay about as far as it would go (it wasn't very far, but it was far enough to reach the spokes of the rear wheel), and I chipped the corner off a piece of foam insulation that just happened to have an elliptical edge that would be just perfect for fitting itself to the seatstay; this provided the extra space buffer I needed, since the magnet needed to be a lot closer to the spoke/sensor (it was made to be on the front wheel). Then I simply zip-tied the sensor and the small piece of insulation to the seatstay, put the magnet at the appropriate spot on a corresponding spoke, adjusted the distance between them, and voila, it worked! I actually used that setup all of last winter to keep my cycling fitness up to a bare minimum...I've found I ride a lot harder if I have something to tell me how hard I'm actually riding.