I think the accelerometer could be used for unlocking/locking the screen.

You would just shake the phone and it unlocks, or you shake and the swipe to unlock screen popups. The approximity sensor would detect when you have the device pocketed, so it wouldn´t unlock accidentally.

Locking by shaking could be done only from the desktop(s), so there wouldn´t be any accidental lockups when using applications.

Intuitive and simple.