Yes that's very doubtfully in the same version, that's what he's saying. 2 different versions, likely 850/900/1900/mhz UMTS on one, and 900/1700/2100mhz UMTS on the other. As for hardware limitations, almost definitely. I can't imagine having 5 bands of UMTS available in 1 phone, I think it'd have to be bigger. You could flash a new ROM independent of radio, and not mess up the coverage, sure.
As for your questions about 850mhz, here's how it goes.
AT&T, in the US: 850/1900MHZ UMTS, depending on the area it will be 1 or the other.
T-Mo, in the US: 1700/2100MHZ UMTS, both active at the same time, one for upload the other for download.
Europe: 900MHZ (I believe) UMTS
Asia: 2100MHZ (I believe) UMTS
So with AT&T, losing functionality of 1 band will limit your reception in certain parts of the country... But don't flash a new radio and it should be fine.
As for carrier, I don't know if any in the US is going to pick this up and sell it subsidized. If not, you're shelling out all $900+ for this baby with no contract.