I tapped into my connections at Nokia Germany in Dusseldorf. The international version will not work on the 1700 MHz band. Nokia produces only 2 hardware models of the phone (aside from some unconfirmed special Asian models). The European (sometimes also considered to be international version), and the North American Version which supports the 1700 MHz band. I asked my contact specifically about the ATT version and the answer I received was that there is only one version for the American market. The European version has the RM-821 ROM and the NA version runs on the RM-820 rom. Whether it is locked or unlock has nothing to do with the rom. The sim-lock is added on top of the main rom.
Thus said, the ATT version will most likely work on the 1700 MHZ band, but, I also asked my contact about sim locks by ATT and though she did not know much about it, since she is not responsible for the NA market, she mentioned that the sim-locks on all Windows Phones are almost impossible to break, unless the provider starts issuing them. Nokia supposedly cannot even take that lock of themselves (at least not in an easily accessible procedure ;-)).
Bottom line, I don't think we will see any unlocked ATT phones within the next 5 months. The international version sold online is not capable to run on the 1700 Mhz band, so if you need the 1700 Mhz band (are not in a reframed area), your only choice is Rogers for now.
I'd love to be proven wrong on this (since I really want the phone on T-Mob), but I have a feeling there is not much to be proven wrong about ;-)