AT&T does publish a list of upcoming 3G locations, I don't have a direct link in front of me at the moment, but if you do a google search I'm sure the search results will yield a link for you that has info...
From what I recall, it's not something that they have readily listed on the front page of AT&T's website, you have to search for it, unless they updated their website..
Here's a link you can check out:
http://www.att.com/gen/press-room?pid=4800&cdvn=news&newsarticleid=25463