Pocket Lock-It

Pocket Lock-It uses the accelerometer to detect when it is upside down (because that's the way most people insert their phone into their pocket) It also uses the Proximity sensor to detect an object. If both conditions are met the screen will be locked, and will let you know it has been locked with a short series of vibrations.

Pocket Lock-It may not be centered on some displays but will still work.

*Important* Because Android now requires special permission to activate screen lock, Pocket Lock-It must be added to the device administrator list. This can be done by:
Go to Settings\Security\Device Administrators, Put a check mark beside "Pocket Lock-It" and then click "Activate"