The suffix "T" version is the handset for T-mobile network. It will NOT give 3G if used on the ATT network. Only 2G(aka EDGE) when used on ATT.
ATT's network uses a different frequency band for 3G than T-Mo, which is why the manufacturers must release separate models for each network.
Best Buy has had the suffix "T" version all this time, because that model was released many months ago, for T-Mo.
The suffix "A" version has now disappeared from the Best Buy site.
Best Buy is saying that the suffix "T" version is for ATT because you can use it to make voice calls and 2G data on ATT. They very carefully avoid mentioning that you cannot get 3G connectivity with that model on ATT.
You would be crazy to buy a data-centric phone, like Nexus S, and cripple it by using it on EDGE only.