Back when I had the opportunity to review the Nexus S, I got Sprint's 4G version. The newest Nexus is also available on T-Mobile, but it's only able to connect to its 3G network there. And that's exactly what's happening with the AT&T version.
Now, Samsung's Nexus S, running on AT&T's 3G network, is ready to be purchased, thanks to Best Buy. In fact, you can only buy it from Best Buy: it's an exclusive offer.
If you can deal with not having 4G connectivity and the fact that your "new" phone is already seven months old, then you can head on down to your local Best Buy and pick one up for $100 on a two-year contract.[GottaBeMobile]