Hi all. I had a similar issue where my clock was disappearing from the low-power mode standby screen, no double tap to unlock, and could not end calls. After reading this thread I realized that a drop (fairly gentle onto carpet while sitting) might have caused the problem.

I then noticed my screen was bulging up a bit. Hard to tell with the case but I could press the screen down about a 1/2 mm.

Having replaced my screen once I figured I would pop the screen off and see what I could see. If you want to do that, watch this: http://www.youtube.com/watch?v=9dxsTGpGRhs

I noticed the white plastic frame around the screen (that holds the screen down as shown in the video) had become detached from the screen glass allowing it to bulge up. This meant the sensor(attached to the white frame) was a 1/2mm or so away from the glass and likely was not getting a good reading. (you migh try simply pushing your screen down to see if the sensor works, though this probably wont work).

So to fix it, with the screen off, I used some CA cement (aka Crazy Glue) and a small brush (tooth pick would work) to reglue the white frame down to the screen.

After reassembling and rebooting, IT WORKED!