My HTC Titan suddenly startet turning off my cellular data (3G) after I turned off the screen for a little while (immediately turning the screen back on and it would still be enabled so it seems to be some kind of timeout).
Workaround for this problem:
1. Turn the phone off and then on again.
2. Go to settings -> Flight Mode -> Turn flight mode on -> Turn flight mode off -> enter PIN code (if asked).
Will add solution to problem if I can find any.