This isn't surprising. Any antenna doesn't like foreign conductors. I can get my N900 to lose bars if I really cup it in the area of the GSM antenna. Honestly, I just tried it. If you cup an iPhone really hard, it will lose reception. The thing is, on the iPhone 4 it's super easy. No cupping needed. There's a video showing that even a finger touch is enough. Usually when you hold a phone, you aren't blocking it enough to influence the signal. But with the iPhone 4 you can do it easily just by touching it the wrong way.