Forum search showed nothing.... I'm not an expert but this makes sense to me. So I was thinking and I realized why there is a lock screen on the iPad. When you press the sleep button, the display turns off, but since the device is on, the OS is still "displaying" the OS, you just can't see it. So the device goes into lock screen mode where it only has to worry about touch, time, and a few other things. If the OS was "displaying" the home screen or an app, it would have to worry about some 17 other tasks (all the springboard, launchd type stuff) or something like that, killing the battery while in sleep.