I've been reading about this signal issue for quite a while now.
As was suggested a few pages back there is a chance that the radio firmware (the software that controls the antenna among other things) is somehow detecting your hand bridging the gap, and from that, adjusts the signal in some way.
Something like this would be the best case scenario as it could be rectified easily with a software update.
There is absolutely no way in hell any of you should 'accept' the fact you can improve the situation by using a bumper, or be happy that you don't use your phone in your left hand. (Take a step back and think about this for a minute if you are of that frame of mind)
I am also quite surprised we still haven't had any official word from Apple on this?
On the flip side, it would appear the iPhone 4 is not alone with this issue, my HTC Desire drops a bar or two when you pick it up(difficult to tell though as it has a weak signal anyway due to where I live) , as does the 3G and 3GS. Its just not been quite as potent as it is now.
However, there is a difference between losing a bar or two, and losing 4/5 bars and your signal altogether!