I use
Touch InCall Screen Tweak.
Basically, the program activates when you make a call. Then, when holding the phone against your ear the screen is off as normal, but when you pull it away to enter digits, the g-sensor polling makes the screen light back up until you put it against your ear again.
With my Innocase making the power button a little... err... less accessible for my already oversized fingers, this program is definitely for the win.