NOt sure about AT&T but Alltel works extremly well. I was sitting in the bottom of a pretty nasty canyon over the weekend up there emailing quotes, relaying messages, etc... the whole time thinking to myself what a sweet office!
Most anywhere on the top you will be able to get decent coverage. I've found verizon works the best up there but at&t should work about the same I would think.
I have tried with AT&T on Monroe and had terrible success. I know of a few places I can call out but only trial and error. If you are in Koosharem you won't get anything.