The problem appears to be in the newly designed antenna array. The metal band that actually wraps the phone houses the antennas, the idea is that the human hand is grounding out or disrupting the signals. Apple insists that this is easily fixed by purchasing a case for the phone.
Personally, I don't use a permanent case for my iPhone. One of the attractions for me and the iPhone was its style without a case. The case completely ruins the iPhone 4's sleek look, making it more bulky.