Always On Proximity allows you to use the proximity sensor in an interesting way

  Always On Proximity is a tweak released during this evening in Cydia, and with its help you can control the functionality of the proximity sensor in iOS. This sensor closes the screen when you hold the phone to your ear to talk to it, but using the tweak Always On Proximity you can also use the sensor to automatically lock/unlock your iDevice. The tweak's functionality is activated by a gesture controlled by the Activator, and the terminal will automatically lock when you insert it into your pocket and unlock automatically when you remove it from your pocket.

Enable or disable your phone's proximity sensor by a simple activator gesture. The screen goes off like it would during a call, so you can just slip it into your pocket and the screen conveniently goes off. While I have tested this tweak for a long time I am not responsible for any issues that you might encounter by using this software. Configure gesture from Activator.

  Always On Proximity is available for free in Cydia's BigBoss repo.