Uh.. Verizon is quite a quality carrier, and AT&T has plenty of bells and whistles, and they have NEVER officially sold the Nexus line. Only Best Buy / Google direct will sell you a Nexus S 9020A that works on ATT 3G.
At any rate, it is quite surprising to me they ditched T-Mobile for the initial launch, given that Verizon is a CDMA carrier and generally goes against the grain with the GSM standard the EU is trying to push, but at this point Verizon has the largest Android following in the US compared to any one carrier, so perhaps it shouldn't be all that surprising.