IPhone won't go dark when I'm talking on the phone?

I think it's ever since I reset my phone (holding the 2 buttons or whatever) my iPhone 3g won't go dark when I put the phone up to my ear to talk on the phone. Which is annoying because now my cheek pushes random things on the screen… Any idea how to fix this?

Look through the settings and there should be a backlight option, or screen lock. Change it to ever 30 seconds or minute of inactivity, whichever works for you