cyclocomputer on front or rear tire?

Typically they are mounted on the front tire.  If you really want to use it for your trainer then you would have to mount it on the rear.  Problem with wireless is that the distance from the rear of the bike to the bars where the computer is mounted tends to be too far for the two parts to maintain communication...at least that's what I found with my Specialized Turbo Elite, so you either need to go wired or have an alternate mounting location closer to the rear (on your top tube maybe) for the unit to work.

i have the cateye astrale 8. Its not wireless but i trail the wire down the the end of the bike and mount the sensors on the back wheel.

No, you don't need to go wireless. The Cateye Astrale 8 (~$25 at performance online) has a single wire that splits into a cadence and a rear wheel pickup. Works great on the trainer.

I have the Trek Incite 11i, which has cadence and is mounted on the rear tire and it works great.

Can someone tell me what 'cadence' means when you guys are refering to it in the previous posts? I just thought people use the computer for mph and distance.

both! i have a wireless sigma unit for regular riding, and an xl cable kit i put on when its on the trainer.

Cadence is the measure of how fast you pedal...RPM for your feet.

A computer that measures cadence has two pickup/magnet combinations, one that goes on your wheel to measure speed/distance, and another that goes on your crank arm to measure cadence.

Cool. Thanks everyone. I think I'll pick up the Astrale 8.