Surely, having a phone with an external antenna which can be gripped directly with the hand will make it more susceptible to RF attenuation than one in which the antenna is internal and comes into no direct contact. Yes, there will be some attenuation when you put you hand around any phone, but designing one with the antenna on the outside is asking for trouble, isn't it?
The other thing that puzzles me is why so many iPhone 4 customers *haven't* seen the problem. These millions of satisfied customers can't spend all their time in strong reception areas - they are *mobile* phones. Even if some customers are more sensitive to signal drop-off than others, it still suggests there might be a fault which only shows up on some examples of the iPhone 4.