coverage vs. capacity
The problem isn't the 3G network coverage, it's the capacity. If AT&T doesn't have the bandwidth on its network to deal with the vastly increased traffic that a 3G iPhone will bring, it's not going to be very fast. My father tried an AT&T 3G device for his computer, just to use in his house, and found the speeds to be not much better than dialup most of the time, though occasionally he would get periods of high speed. This despite living in an area with 3G coverage.
The problem isn't the 3G network coverage, it's the capacity. If AT&T doesn't have the bandwidth on its network to deal with the vastly increased traffic that a 3G iPhone will bring, it's not going to be very fast. My father tried an AT&T 3G device for his computer, just to use in his house, and found the speeds to be not much better than dialup most of the time, though occasionally he would get periods of high speed. This despite living in an area with 3G coverage.