Even though you don't have service with it, you still must de-brand it to get updates immediately when they are made available. As of right now, that is 7008, pre-nodo. I would not anticipate NoDo being available until AT&T is done testing it. One they are ready to launch, I would assume that de-branded devices would be among the first to get the update.
I have a Quantum that does not have GSM service yet.
Since plugging the device into Zune checks for updates, the updates are done through Zune, and our devices are still AT&T branded whether or not they are active, I would be willing to bet that one does not need GSM service to receive updates.