I have an iPhone 4S on AT&T (I'm located in Denver). In a variety of locations, with moderate to low coverage, seemingly more often with calls that I receive than that I place, I answer the phone, and I can hear the caller absolutely perfectly--the signal seems very strong. But they cannot hear me at all. No intermittent in-and-out: they hear nothing at all from me.
I just upgraded from an iPhone 3G on AT&T and never had this issue. But I can't say whether it's related to the iPhone 4S, or iOS 5, or an interaction of one of them with AT&T's network.