AT&T is too iphone-centric to have a serious Android phone. Although it wouldn't hurt having one, many companies do not see much profit from adding one.
You may be able to get an unlocked GSM phone to use on AT&T's network. Way back in 2002 before Steve Jobs invented the smartphone I had a Sony Ericsson P800 I bought direct from Ericsson (full touchscreen {the keypad flips down}, all the apps I could hope for, email support, camera phone, video camera, cut and paste in the first release - I loved that phone!). I've seen rumors that HTC is making an Evo for GSM called the HTC Ace. Obviously you'll probably pay full retail and on AT&T you won't get Sprint's 4G network, nor Verizon's upcoming LTE, but you may at least get a 3G "Evo" by just swapping your SIM over.
That said, any reason to be tied to AT&T? I can understand not liking Sprint, but depending on what you want the Evo for the DroidX may be a decent replacement on Big Red who seems very supportive of the Android cause. If you need the front-facing camera you may be out of luck and HTC has a more open architecture if you're a techie, but speedwise you won't get 4G on AT&T for quite some time and Verizon said they won't be releasing LTE (4G) smartphones until next year.