I would say there's a 99.99% chance that AT&T has a roaming agreement with all gsm providers in the United States. When I was in Nebraska, I roamed outside of native AT&T coverage area and was using Vaiero wireless. I roamed on them for no fee whatsoever for about a week. They didn't allow me to use their 3G network, so I was forced to stay on EDGE.
A lot of providers have a rule that if you roam for a certain percentage of your monthly allotment, they can cancel your contract since that violate it. You should ask AT&T what regional providers may be in that area and if they have a roaming agreement with them. 10 years ago, when providers still had regional plans and roaming was dangerously expensive, I would say watch out. But nowadays, unless you plan on camping for the next few months, I would find it very hard to believe if you said you were charged for roaming in the US with a US carrier.