By default, iOS will lock the screen and disable the touch sensor if there are no touch events for a specified period of time. Depending on your application, there may be times that you need to keep the screen from locking. For example, if your application is primarily accelerometer driven (such as a game), there may be minimal touch events to keep the screen from reseting its internal timer.
You can disable the idle timer through the idleTimerDisabled property of the shared UIApplication object:
// Disable the idle timer
[[UIApplication sharedApplication] setIdleTimerDisabled: YES];
// Or for those who prefer dot syntax:
[UIApplication sharedApplication].idleTimerDisabled = YES;