It seems another XDA has found a new way to control your backlit screen while calling. The actual feature is only available on stock T-mobile HD2, so sorry Europeans and everyone else, I guess you need to flash a ROM.
While in a call, you simple use two fingers and tap the screen, and that should turn it off. This is just a simple alternative to having your device pressed against your face. You can also turn your screen back on with a quick shake, and that should help you get on with your day.