Even though this thread was from last summer, I thought I'd chime in if someone else was wondering about this. I know several ppl with AT&T up here and even in remote areas they get decent coverage. I kept my Verizon plan when I moved up here and since they have an agreement with ACS (the largest AK cell phone company with the best coverage in the state) to use their towers, we get really good coverage without roaming charges. I hear Verizon will be moving up to AK sometime in the future. Data coverage is decent too, even a long ways from the metro areas.
I've only been on the Kenai peninsula, Anchorage area, Matsu, and up the Glenn and Richardson highways so far. Except for the scattered expected no-service areas b/c you're down in a big hole in the middle of nowhere, I've been able to use my phone in most areas along the highway corridors.