Every once in a while there are some serious issues with the calibration of the touch screen of my HTC TyTN. The calibration is off by 3-4 millimeter and sometimes even by more than one centimeter. It typically happens when I switch the device on after keeping it in my pocket for a long time.
It turns out other people are noticing the same problem. There are a couple of suggestions to prevent/fix the issue. They range from keeping the PDA screen covered, over untightening some screws to rubbing your thumb over the screen.
BTW: There is a handy shortcut that will take you directly into the screen calibration utility: hold down the center button in the middle of the direction pad, then hit the messaging button.