Spencer Webb of AntennaSys (an "antenna design" firm) knows the science, though, and he's shared a little outsider wisdom on how the iPhone's metal actually works.
Webb starts by talking about the design of cell phone antennas in general. He says that most antenna placement these days is based on FCC rules about how much energy a device can send into a user's head; that's the reason most antennas are at the bottom of phones nowadays. Unfortunately, our hands are usually on the bottom of the phone, and the FCC doesn't test energy absorption through the hands, only through the head. What's the ideal way of holding your phone? You should hold it by the top, as if it were hanging in midair.
Where does the iPhone 4 fit in all of this? Webb says the antennas in the new iPhone are on the sides of the phone, which means that, yes, there's potential for your hands to block energy. But there's always the potential for that, and Webb says that if you put your phone somewhere away from you and use a Bluetooth headset, none of it really matters anyway.
What's the final verdict? Webb himself preordered an iPhone 4 to replace his original iPhone. Sure, the antennas might not be perfect, but given the quality of the phone (not to mention the experience), "good enough, is good enough."