happened to me too. sent back to ATT was a bit nervous because I unlocked the bootloader, but lo and behold no issues (like always)
it got progressively worse, touch test showed the center wasn't responding, then more and more and more until I couldn't touch a thing (I had since switched to a backup)
got my replacement, all is well.
I did notice that this seemed to have happened after I started spending considerable amount of time playing the game "R-Type" obviously the app didn't do it, but it has touch controls that require you to keep your finger pressed on the center of the screen for a prolonged time, just food for thought.