People, this is a wonderful engineering trade-off (and I've done some RF engineering at a major electronics company). A phone's antenna gain is affected both by how near it is to the counterpose and the users hand. (The counterpose is usually a mix of the circuit board and other antennas.) On most phones, this antenna position is an ugly compromise, partially inside, very near the circuit board which also interferes with the signal. Or the phone is bigger to make room for the case to get the antenna farther away from your hand. On those other phones, you can do very little about this non-optimal antenna location.
On the iPhone 4, the antenna is as far from the counterpose as possible for the given battery size. (You can't go any farther than the outside edge of the largest dimension of a rectangle without one of those ugly stick up antennas that really old cell phones used to have.) This is optimal engineering.
Of course, this also puts the antenna as close as possible to your hand. But now, you, as a user can do something about it.
You can be in an area with a cell signal so powerful that you get 5 bars even with the antenna partially shorted, loaded and covered (which is why some people report not seeing the problem).
You can hold the phone with your hand far away from the antenna and the antenna insulation gap.
Or you, instead of Apple, can choose to make the phone bigger by adding a bumper or other case which moves your hand farther from the antenna. If you have a strong enough signal, all it might take is a strip of clear insulation, maybe something like screen protector material for the bottom left edges of the phone.
You choose.