What bike computer (speedometer) to get for mounting on the rear wheel? I'm looking to get a bike computer, primarily for the speedometer function. I use my bike on a trainer most of the time, and I'd like to get "speed" from that, so the sensor needs to be on the rear wheel.

I'd prefer not to spend too much, but my budget is pretty high. Right now, the only feature I want is speed, but I'll learn to use any other useful features. 

I've read this question, and gotten some interesting data, but I don't really care about whether the unit is wired or wireless. It just needs to reach the back wheel and be reliable.
I've decided to go the cheap route - I got a Cateye Enduro 8 (which is wired, but not long enough) and I'll modify it to extend the wire.

If I start using my bike more (or I break the Cateye) I'll look at getting something like the Garmins @zenbike mentioned.