[hoopz]

So you know when you're calling someone, the screen goes to sleep (goes black) but when you take it away from your ear it comes back on so you can tap end call or keypad or whatever? How does it know to do that? That is so cool. I've noticed that I can move it away from my ear and as long as I keep it an inch or two from my body it will stay asleep, but the second it gets too far away it pops back on. I love iPhone.

[Hackme]

just to the right of the speaker on the top part of the phone there's a sensor that responds to distance it is from something. Im pretty sure it measures the amount of sound that bounces back after being sent from the speaker.

[ShadyBeagle]

There is a proximity sensor inside of the iphone you can watch a video at http://www.apple.com/iphone/features/index.html#sensor