I live in Johnston, suburb of Des Moines, I have not had any signal issues yet. I recently dropped verizon for AT&T and the IPhone. I absolutely love it.
I just took a trip to the Rockies to go snowboarding. We took the amtrack and stayed at the base of the mountain. Never had any signal issues through the entire trip. The best thing, I got back Monday and saw that the 3G network was active in Des Moines.