We've received only one report, so take it with a hill of salt: A Gizmodo reader claims that, after Apple replaced his iPhone 4, he can't reproduce the sharp loss of signal. He says that the hardware seems slightly different:
I've been following all of the iPhone 4 stuff since I got mine the day before release. I was able to replicate the signal issue. I also had the proximity sensor issue, causing inadvertent mute button pushing. There were a few other software issues I was experiencing so I consulted Apple at the Fifth Avenue store in New York. They replaced my phone. The diagnostic showed that the OS was corrupt and certain utilities were failing. [They claimed that]all phones with a proximity sensor issue were being sent back to Apple for further study.
Well, when I got the new phone it was different. It was different hardware. The black [plastic]bezel isn't as black on the new one. I couldn't see the proximity sensor at all on the previous iPhone 4, now I can. The stainless steel band on the new phone is less 'steel-y' and more matte. I've also tried to replicate the signal drop and failure. While I can't say for sure that it is entirely fixed, there is certainly huge improvement. I'm guessing they coated the steel with something, took some black out of the bezel and sent them out without saying too much about it. I also think Apple is willing to warranty a phone for any reason except the signal issue.
The guy next to me said that he had to press the home button several times before the phone would come back from stand-by. This happened once. The Apple tech ran the diagnostic, everything was fine. They still replaced his phone. I think they're doing a 'silent recall'.
We doubt Apple would do something like this. While it would seem smart to silently modify the production process - coating the antenna was the solution suggested to us by a chemical engineer - and only replace the units of those who actually protest about it at the store, it would not be good for the company if something like this gets uncovered. Apple hasn't answered our questions about this subject.
Again, this is a single isolated reader report and we have not been able to confirm it: Following his claim, we exchanged one of our units, but we didn't get any different hardware back. The antenna reception problem was still there.