Ok this is hilarious to Apple haters. I used a Motorola H17 bluetooth headset for about a year with my iPhone 3GS and it was flawless and I loved it. Shortly after getting my iPhone 4 on release day I noticed that callers couldn't understand me and told me I was garbled and awful, although I could hear them perfectly. This was only intermittent, but I [incorrectly] assumed that the headset was the problem and I threw it away (I know, I'm dumb, I didn't see this thread then obviously). So then I bought a Jawbone Icon. Still noticed people complaining about my sound quality, returned it. Then bought a Sound ID, same problem, returned it. Finally picked up a Plantronics Voyager Pro, which by the way kills all of those other ones in terms of sound quality both ways. But I still had intermittent problems, and only then did I hit the forums to realize that ios4 was the problem! So I tossed a perfectly good Motorola H17 and went to the store a bunch of times for no good reason! Although actually I am glad I got to do head-to-head comparisons of the top-end Bluetooth headsets out there, and the Plantronics Voyager Pro stomps the Jawbone Icon and all the others, in case anyone wondered. But back on topic, it would be nice if Apple would fix this! I would also give them partial credit for just acknowledging it at this point.....I did my part and called it in and reported it as feedback, now just have to wait.
My social engineering solution right now is this: when I get a call and I want to use my bluetooth, I answer and then as I'm saying "hello" I hang up (tee hee). Then I turn on my headset, call them back, and blame the dropped call on AT&T. Clearly not a really good strategy, and I hope Apple fixes this problem ASAP!!