I've been using my iphone for less then a week now, and I can say that the sound quality is probably as good as I could expect being that AT&T is the carrier. I seriously wonder how people can be 'amazed' with the sound quality with AT&T's service. The 'wobbling' sound I get on ringing sounds as well as voice fluctuations really drives me crazy. Of course, I notice those on my Blackberry as well. As far as I am concerned my 3 year old verizon motorola sounds better, but I know it's the network, not the phone.
I have unlocked my iPhone and using a t-mobile sim in it. I am much more impressed with the sound quality. Again, I am not slamming the iphone, I'm sure if they had made it for Verizon it would have been able to hit it's pinnacle, but it's very good for what it has to work with.