You're exactly right. The unlocked GN which is not an AT&T phone works with GW and the One X which is an AT&T phone doesn't. The reason the GN works is only because AT&T allows it to, I'm guessing under pressure from Google, MasterCard, Citibank, or some combination of the three.
Google can't make AT&T allow GW support on any of their branded phones. The international One X has a security token, is unlocked, and still can't connect to GW because AT&T's blocking it. And why would Google have updated GW for a phone that's not even out yet and not one in people's hands if they did intend to provide support for it? Sprint provides support for GW on all their NFC equipped phones. Sprint was also the carrier launch partner for GW in the U.S.
I think we're all pretty much screwed as far as being able to use GW on AT&T.
Also, it's not a case of at&t blocking Google Wallet traffic on their network, as you had previously suggested. Wallet does not actually need a data connection to complete transactions. The application simply checks the device ID and the sim network ID.