I can report couple instances when I have seen this happening too, and it had nothing to do with lock button.

Last time was yesterday when I used MaemoMapper. Suddenly the unit become unresponsive to the touch screen touches (I still could hear the click sound when touching the screen). The hardware buttons worked and using them I was able to close the application and reboot the tablet. After this it worked normally.

So, since hearing the clicks the screen does recognize the touches, but likely the calibration got messed up so that my presses were recognized "outside the screen".