The same sensor that the Iphone 4 is having issues with. The sensor there is not turning off the screen when it's close to your face so you mute and hang up on people.
Apple says it's a software bug they are fixing.. However, they also said the antenna problem was a software bug and we all know that story.
I was talking on the bluetooth and the screen was mostly locked. The only portion that seemed to not lock were the obvious ones to enable/disable bt, end call and what not. The rest of the display didn't appear to do anything. I didn't try to change screens but the display did go to sleep during long calls.