AT&T and Verizon use different wireless technologies. The radio in the Droid 2 isn't compatible with AT&T's network.
Supposedly there's a Droid 2 world phone coming soon, and that would have a compatible radio, but that doesn't mean it would necessarily work. There might still be some kind of carrier lock-in.
If you're looking for a solid Android for AT&T, I recommend the Samsung Captivate. A friend of mine has it and it's a fantastic phone. No keyboard though.