Anandtech has done some testing to try and find out what's happening with iPhone 4 signal attenuation and related antenna reception problems, and come to two equal yet opposing conclusions:
Holding the iPhone 4 without a case, in your left hand, crossing the black strip can result in a worst case drop of 24 dBm in signal. [...] The fact of the matter is that either the most sensitive region of the antenna should have an insulative coating, or everyone should use a case. For a company that uses style heavily as a selling point, the latter isn't an option. And the former would require an unprecedented admission of fault on Apple's part.
And here's the flip-side:
iPhone 4 performs much better than the 3GS in situations where signal is very low, at -113 dBm (1 bar). Previously, dropping this low all but guaranteed that calls would drop, fail to be placed, and data would no longer be transacted at all. I can honestly say that I've never held onto so many calls and data simultaneously on 1 bar at -113 dBm as I have with the iPhone 4, so it's readily apparent that the new baseband hardware is much more sensitive compared to what was in the 3GS. The difference is that reception is massively better on the iPhone 4 in actual use.
Dieter is struggling with a lot of this while traveling back from the Samsung Galaxy S event in New York yesterday, what with AT&T's historic iPhone network problems and now this added uncertainty of how iPhone 4 is held has him speechless (in several ways).
We can put a case on it and hope for the best, or hope Apple will quickly push out an iOS 4.0.x update that alleviates it, but the layered and varied uncertainties surrounding what for many is their primary method of connecting and communicating is no doubt becoming untenable for some.