When the Nexus One originally launched on January 5, 2010, it was only compatible with T-Mobile 3G, although an AT&T-compatible model was made available just a couple of months later. Now it appears that the Nexus S is going to follow in the One's footsteps. The first piece of evidence is a device with the model number GT-I9020A that was just spotted on the Bluetooth SIG's website. The European Nexus S is the GT-I9020 and T-Mobile's model is the GT-I9020T, so it stands to reason that the GT-I9020A would be headed to ATT.
Our second bit of proof that an ATT Nexus S is on its way comes from Canadian carrier Mobilicity's CEO. The exec recently stated that Bell, Telus, and Rogers would be getting the Nexus S some time in March, but there's currently no version of the device that would work with those carriers' 3G networks.
Considering that an AT&T 3G-ready Nexus One was eventually made available, I wouldn't be surprised to see an ATT Nexus S arrive some time in the near future. It's not clear exactly when the device is going to arrive, although after looking over all of the information we have now, a launch some time between the middle of February and the end of March sounds about right to me. When you add the Nexus S in with the Atrix 4G, Inspire 4G, and Infuse 4G, I'd say that AT&T is building a pretty solid Android lineup post-VeriPhone.