My girlfriend and I recently switched to AT&T, they have great coverage in my area and I like gsm. The only problem is the lack of android phones. I don't like the backflip and I have some concerns with the nexus one. What I'm really looking for is a list of unlocked phones that would work on AT&T 3g and where I could purchase these phones. I would prefer to have a higher end android phone since phones like the g1 and hero are becoming very out of date. I hope there are some good phones out there cause I have an iPhone and I'm really itching for a good android phone!!!
The best your going to find on AT&T currently is the Nexus one by Google. The BackFlip is severely crippled. I've had mine for about three weeks now and while it does have a few bugs, most of them are software related. All of the touch screen issues I read about before buying mine are legit, but in real life they don't mean a whole lot. So far my AT&T N1 has been great.
FYI, I asked the same exact question you did when I first started looking.