Is this a serious question???
Of course the screen is on how the hell do think the buttons are pressed if the screen isnt on!
Sorry, but the screen does NOT have to come on to have buttons activated. You don't even have to touch the screen with your cheek.
When I got my latest iphone 4, I turned up the brightness all the way - so if the screen came on, I could see it easily. I also used it while in front of my LED monitor, which was black (so I could see it like a mirror). Finally, part of the time I even had the iSight camera on so I could go back and watch to see what happened if I activated a button.
During this test, I held the phone normally which, for me, means the phone is on my ear but angled away from my cheek, but I was watching it and recording it sometimes just to make sure.
In less than an hour of talking to a friend, the call was ended 6 times and the mute button was activated once. To make sure the call ending was the phone and not the network, I went to the Apple store so they could look up my phone record for dropped calls (if you end the call by pressing the button, it doesn't show up as a dropped call) - just a normal end. They showed zero dropped calls so it was the phone problem.
But the screen NEVER came on during this test. Yes, the screen sometimes does come on as well, but not on this test and no hitting buttons with my cheek.
Either there are two problems and mine isn't the sensor (fairly unlikely since it was an issue with both disconnecting me and turn on Mute), or there is something where the buttons can be activated in certain situations even when you can't see them.
As for not hitting it with my cheek, only thing I can guess is that there is still an electrical affect anyway - just as you could walk up to the old rabbit ear antennas on TVs years ago and change the quality of the signal if you were close enough, but not actually touching anything.
I know of at least one other person who had buttons activate while the screen was off.
