I travel throughout the US as well and have not had dropped calls. It is just not an issue with me. I understand that it does happen to a lot of people. I have AT&T specifically for the reason that they have great coverage in coastal areas(where I do all of my work). I have tried everyone else, but AT&T covers these areas better.
I second that (city coverage issues) when I travel to NYC or SanFran on business my iphone is little more then a phone. If I can make a call, I usually can't get a data connection. When in SanFran for the GDC (Game Developers conference) a bunch of us with iPhones had no Signal, while AT&T Blackberry were fine. This is standing in one place. Overwhelmed 3G and not falling back to Edge was the problem.