I can't seem to get any 3G service on my iPhone, and haven't for about a week now. Please help. I know that we have 3G service in this area, because I'm holding another iPhone in my hand, and that one gets 3G service just fine. I know that 3G is enabled, because I go to Settings -> General -> Network, and "Enable 3G" is turned on, and just says "No Signal." When I turn off "Enable 3G" I can get on the Edge network just fine. I don't get it. I already tried reseting the iPhone (holding down the home sceen button and the power button), and that didn't work. I tried upgrading to OS 4.0 and that didn't work. I even tried going to Settings -> Reset -> Erase All Settings, and even that didn't work. I am completely and totally stumped, and I am out of ideas. Help!
Call AT&T and make sure your 3G data plan is in order. Sometimes, for unexpected reasons, data plans can be dropped off. You can also try opening the SIM slot, taking the SIM out, and putting it back in.