can i ask why? temperature is important to know before you go out, but once you're out there what's the point? if you're warm, shed a layer...cold, put on a layer. i don't need a thermometer to tell me if i'm warm or cold. plus the majority of mountains post base and summit temps and usually have a thermometer at the top of the lift.
on the other hand, an altimeter watch would be sweet for riding. keep track of elevation, vert traveled, etc. look into it...pretty sweet.