AT&Ts 3G rollout decisions are based on multiple variables. The first variable comes down to technical limitations on the areas they are rolling out. Any of the areas which were originally Cingular and had 850Mhz spectrum with them which AT&T then became owner of seem to have been the first and easiest to move to 3G. Given that AT&T is trying to move to 850Mhz in general for rollouts, this makes sense as you'd only want to move to 1900Mhz if you have no 850Mhz spectrum available. Anyone interested in the general areas of who owns what/where for spectrum can see here: http://www.cellularmap.net/regions/
After that, its a combination of market size, users and ROI. Once they have the larger cities completed I'm assuming they are moving onto phase II which is to hit the cities (50K + sizes). Of course there is also politics involved which are usually based on local state requirements for cell towers, spectrums, etc.
A positive for the smaller markets maybe the Verizon/Alltel merger though because if Verizon moves the Alltel areas onto the Verizon network and EV-DO Rev A upgrades it may force ATT to move up their schedule to upgrade those markets to 3G in order to stay competitive. This is the scenario I'm hoping for in my area. (54601)