Quadband GSM is one thing, 3G support is another.

Nokia has released most of their N Series smartphones unlocked in the US with AT&T 3G frequency support. They have not released any parallel versions with T-mobile 3G frequencies.

My guess is that if this device will remain without a carrier subsidy, they will only produce it with support for one GSM carrier's 3G network and not the other.

On the other hand, since Maemo on a phone is thus far unproven--Nokia may be going with support for only the T-mobile band since it is the smaller carrier in terms of number of subscribers. I suppose that it is less of a risk to "live test" this device on T-mobile, rather than potentially alienating AT&T's larger user base if something does go wrong.

If all goes well, I would presume that more maemo phone devices would be forthcoming that would be usable on the AT&T 3g network.