The current explanation that I've noticed floating around is that there is coating of some kind (obviously dielectric in nature) that is supposed to be on every single iPhone 4 before leaving the factory and the coating is applied only to the antenna bands/chassis components.
The current explanation says that the coating process wasn't 100% successful for every single iPhone 4 that's been shipped so far, hence the potential reason for why some folks have this issue and can replicate it quite easily and others cannot make it happen at all no matter what they do.
There's a report from one ABC TV affiliate of a customer that had such an issue with his brand new iPhone 4, took it back and they replaced it on the spot with one that did not have this particular issue even when comparing both phones in the same store in the same manner and obviously off the same cell site.
That would lend a lot of credence to the coating explanation. What it doesn't explain is why Apple neglected to mention such a coating and also that this coating - as all coatings do - will obviously degrade over time and leave the phone in a condition where this issue is just as prevalent as it is right now for anyone with an iPhone 4 that potentially doesn't have said coating.
While no one from Apple has said in any official capacity that this "coating" exists and could be or most likely is the reason for all this hubbub right now, I'm leaning towards it being the explanation.
Regardless, it is a very real issue despite some folks not believing it is. You may simply have a "coated" model and should consider yourself fortunate to have one because many others apparently don't. Just because you may not be suffering from this issue, others are, so take that into consideration before posting.
As for Steve Jobs' rather pathetic answer of "Just avoid holding it that way." is a slap in the face to every iPhone 4 owner or prospective buyer and he just basically told every one of you "Hey, I know this stuff better than you, the phone works, you're the one with the problem" like he does every time Apple gets themselves in something they don't care to deal with.
This one isn't going to get swept under a rug, Steve. Deal with it.