Consumer Reports has run lab tests and confirmed what many real-world users likely already know -- in areas with weak signal, touching the iPhone 4 external antenna band at the bottom left side can cause a loss of voice and data connection.
We reached this conclusion after testing all three of our iPhone 4s (purchased at three separate retailers in the New York area) in the controlled environment of CU's radio frequency (RF) isolation chamber. In this room, which is impervious to outside radio signals, our test engineers connected the phones to our base-station emulator, a device that simulates carrier cell towers. We also tested several other AT&T phones the same way, including the iPhone 3G S and the Palm Pre. None of those phones had the signal-loss problems of the iPhone 4.
A piece of duct-tape placed over the offending area can prevent the problem, according to them, and they repeat that a case can likely prevent it as well (though they haven't tested any in the lab yet).
The signal problem is the reason that we did not cite the iPhone 4 as a "recommended" model, even though its score in our other tests placed it atop the latest Ratings of smart phones that were released today.
So their bottom line is iPhone 4 is terrific for its display, camera, battery life, gyroscope, FaceTime, and other features, but if you have the antenna issue, it's a show stopper and they believe Apple needs to come up with a free fix before they'll recommend it.
[Consumer Reports -- Thanks Phil!)