The signal / antenna problem is the reason. They say the problem is not AT&T or faulty signal bars. It the placement of the iPhone 4 antenna. They tested it under laboratory conditions and with multiple iPhones and other phones. Only the iPhone had a weak signal.
We did, however, find an affordable solution for suffering iPhone 4 users: Cover the antenna gap with a piece of duct tape or another thick, non-conductive material. It may not be pretty, but it works. We also expect that using a case would remedy the problem. We’ll test a few cases this week and report back.