The HTC Touch Pro 2 (review) is among the very few Windows Mobile device that has a proximity sensor (the other being the LG Incite). The purpose for the proximity sensor is to turn off the display when you’re on a call to conserve battery. Then, when you pull the phone away from your face, the screen goes back on so that you can view you calendar, add another caller, etc.
This short videos demonstrates the operation of the sensor. It works pretty well, and is actually more sensitive than that on the iPhone.
For those of you wondering how to get a Touch Pro2, you can either spring for an unlocked euro version (from Clove, among other retailers), or wait for your respective carrier to begin selling the device at subsidize prices. Most major global carriers will carry it. If you’re in the US, here are our best guesses on release dates, but keep in mind that they are indeed guesses: T-Mobile, August; Sprint, September; Verizon & AT&T, October-December.