I like the hall idea, but I actually found a phototransistor and IR led combo which is making a dandy photogate (I'm having a couple of interrupt deconfliction issues, but it appears to be working pretty well). The wheel that the motor turns via belt has three large cast "spokes", so that should do just fine in terms of RPM sensing.
dc42: The weird thing about this is that the pot isn't being used as a variable voltage divider. Instead of the pot being connected as high-wiper-low, it appears to be high-wiper, with low disconnected, a variable resistor rather than a voltage divider. I know--I'm not sure what I think of that either.
Worse, while the high-to-low voltage (where the pot should be connected) is only about 18v, each pole is at over 100V with respect to case ground! This resulted in a mighty spark when a lead contacted the case of the power supply I was using (and borked the power supply's low-end output section; think I need a fuse replacement).
So I'm not sure exactly how comfortable I am with this digital potentiometer, but it's the cheapest way forward that I see, so I'm going to give it a go. I just may let the blue smoke out of my Arduino if I'm not careful.