Europe (and most of the world) does not use the same 3G bands as AT&T and pretty much no modern phone (Nokia E7/N8 aside) will have both AT&T and Euro bands. They're both oddballs compared to the rest of the world.They will give you with no contract a similar device. This has always been the case when they are forceably moving you off of a technology that you are using.
And yes you can use an unlocked Eu model of jsut about any phone on AT&T 3g since the frequencies are the same where t-mobile uses an oddball frequency compared to the rest of the world.