I have 2 issues I just ran into yesterday. I have a 3GS unlocked/jail broken with cydia.
My auto lock feature has quit and when I'm talking on the phone and pull the phone away from my ear , say to look up something the screen doesn't wake up just stays black. It stays like that till the person on the other end hangs up then I need to hit the sleep button then the home button to get the screen to wake up. Also if I hit the sleep bottom while having a conversation it will hang up on the person.
The programs I downloaded were, chatpic, fakeclockup, activator and lockinfo, plus a few other themes I tried but deleted. I went and deleted all those programs re-booted and still no auto lock or the other feature.
Any ideas?

1. Restore it to factory default and set up as a new phone - don't install any apps and check if the problem occurs.
2. Unplug tape nr 3 , check its sockets/connectors , blow it/clean them.
3. Proximity sensor broken - it's on flexible ribbon marked "3" together with light sensor and pads for speaker - it's a cheap part (it my opinion this is your problem)
4. Mother board fault.