This is the only android phone that has done this for me. I also love when you pull your ear away the number pad automatically comes up. Which is really awesome.

 

The Nexus One does this as well.

 

My Evo does this too.

 

Htc desire does it

 

Virtually all smartphones with touchscreens do this via an infrared proximity sensor. They have to do it otherwise each time you hold it up to your ear/face the phone would recognize any contact on the screen just like a finger press. It's called face-dialing and the iPhone 4 had issues with it due to a faulty sensor. A few of the cheaper phones don't have a sensor, but they also don't react the same to touchscreen contact during a call.

You've probably heard of pocket-dialing too, where the phone is in your pocket and wakes up from a press of the power button. But on some phones there is no unlock screen so you could end up calling Tibet for hours without knowing.