Alexander --
From personal experience, I can tell you that nearly everyone on this thread who went through a phone exchange, at one point or another, came to the same conclusions as you - hoping that it might be software.
However, there are a number of things at play here - the speaker in the handset, the speakerphone, the proximity sensor, the ambient noise sensor - ALONG with the software. Those of us with replacement phones that have had success, have the same software (at least in regards to iOS5) as you and everyone else. If it were only software, then those who replaced their phones should never have success. Nor should those who have never had a problem, be without problems. The iOS has not been updated yet except to developers, so we are all on the same page in regards to software - the only variance is hardware and service provide - and the broad conclusion is that it is a problem that is independant of cell carrier.
I'm only trying to tell you that we all 'hoped' it would just be a software fix. But in the end, it does indeed appear to be a hardware issue. I have not seen a report yet that a restore, an update or some other software fix has resolved the problem (including some who have reported on this thread who are developers that have access to iOS 5.0.2.)
There is always the possibility that there is some problem with the hardware that MIGHT be fixed by making alterations to the software. But keep in mind - Apple has yet to acknowledge this problem in any way - and the only people who have found relief from the problem are those who exchanged and were lucky enough to get a functioning phone in the process (myself being one of those.)
Time is running out for those who purchased on launch day. But I can tell you that since my replacement arrived last Friday, I, like a few others, have been 'abusing' the phone - making one call after another. making long phone calls, using Siri then making a call, using bluetooth, using the speakerphone, using earphones, switching between bluetooth, speaker and handset - and I have yet to make the phone repeat the problem that I originally had - going on 6 days of use now.
I'm sorry, but if this were strictly software, this problem would be showing up on a majority of phones. As it is, while there have been a large number of us - compared to the however many millions they've sold, we are still in the minority. As I said, the problem MIGHT be fixed by software - but do you want to take the chance? My advice is get a new phone. Replace until you receive one that functions. Otherwise you're rolling the dice.
Hey, MichaelB, just a quick shout out to say great work on attracting attention to the problem. I wrote several of the blogs myself, but I think your stubborn persistance caused several of the postings we've seen. My belief at this point is that Apple is certainly aware, but their standard policy is to remain mute until they have a solution - and with this problem being so intermittant, I would bet they are having a hard time nailing down exactly where the failure is. Or, if they know what it is, they are trying to find a way to remedy with software to avoid a 'recall' of defective phones. OR, they are just going to quietly continue to replace the phones of those who take the initiative to complain and hope the problem clears up. Given the fact that this thread (and I'm sure it's not the only one) continues to grow, almost by the hour now, it's unlikely to just disappear. We will see. Fortunately for me, my problem is resolved, at least up to now. Good luck to the rest of you that are still figuring this out. I will keep an eye here and certainly report back if another problem arrises, but at this point, replacement was a complete fix for my issue and it appears to be the only remedy so far. -- Ted