Why is my iPhone 3GS proximity detector not working? Whenever I use the phone on my iPhone 3GS, it detects my face's proximity and dims the screen. However, once the call ends and I take the phone away from my face, the screen does not brighten again until I press the power/wake button, at which point the screen returns to normal.

Why is this happening? How can I fix it?

Some basic info: I'm running iOS 4.1, jailbroken.
I don't remember ever reading about a problem with proximity sensors in iPhone 3GS running iOS 4.x. The well-known problem was with iPhone 4, and it was fixed in iOS 4.2.1.

I guess you will have to restore your iPhone, either to verify that your problem was caused by some rogue app you installed via jailbreak or to take the phone to an Apple Store.