I have my n900 running on TMobile USA and I can confirm 3g does work. I only pay for the 10$ "Dumb phone" plan.

Edit: I'm dumb i read that wrong. It only works as EDGE on ATT. Which is either 2 or 2.5g(i cant remember which). So to confirm for sure on ATT with n900 you will NOT get 3g.

I can say though i really recommend TMobile. Their prices blow everyone else away and i have not had a single problem with my service.