After updated to iOS 6, my iPhone 4S screen cannot auto-sleep consistently. When it won't sleep automatically, the screen will stay awake consuming battery life which is like a nightmare you do not want it to happen.
I have searched through the Apple official discussion and found many iPhone 4S owners are having this issue after software upgraded to iOS 6. Some claim Apple iPad 2 is also affected. The root cause of this issue is the iOS AssistiveTouch feature. The screen will not auto lock after tapping the AssistiveTouch's home button.
The temporary solution is after tapping AssistiveTouch home button, swipe or touch anywhere on the screen, it will start countdown to auto lock. Not the best solution as we might forget to swipe the screen at times but it works.