Can I mount a wireless speedometer on the back wheel?
 

I do a lot of riding on a trainer and would like to use it indoors?

Sure. Why not. My wireless speed sensor is on my back wheel. Sensor is attached to the chain stay (on the non-chain side) and the sensor is on a spoke. Works great.

The only problem you might run into is if the transmitter isn't powerful enough to be picked up by the computer that far away from it. If you spent money on a good computer though, that hopefully wont be the case.

Cateye, Trek, and Garmin ($$$) sell computers with the transmitter on the rear wheel. Some of these combine cadence and wheel speed into one transmitter.

Some of the less expensive transmitters might not be quite strong enough. Some of these require "line of sight" between the transmitter and the head unit. They might work, if you are lucky.