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'.
It would be nice for Apple to publicly apologize for the mistakes they made with the iPhone 4, both their attitudes and non-acknowledgment of the problem in the first place. And to then offer free replacement to people that actually have both of the problems, being reception and proximity. I do not see Apple doing this in the near future, but I do hope that they are still investigating the cause of the issue and not relying on a 'software' fix.
"Necessity is the mother of invention" is a silly proverb. "Necessity is the mother of futile dodges" is much nearer the truth. -- Alfred North Whitehead