Mmm...have you tried Incall Screen tweak?
It turn the screen off when you have it too your ear, and on when you take it away to maybe hit numbers on your keypad
Here's the link to XDA:
http://forum.xda-developers.com/showthread.php?t=488991
I've also attached the cab
Oh and this app doesn't use the light sensor b/c the light sensor doesn't work while the screen is off.