My theory (and it's just a theory) is that the T-mobile bands decision was based on 2 factors. 1) T-mobile is a big-time player in Europe, and thus getting cozy with them makes good business sense for nokia's plan to push this platform as the high-end offering to providers and 2) Supporting T-mobile's U.S. 3G bands required the addition of only a single frequency above and beyond what is necessary for Europe...allowing a single tri-band 3G radio SKU to be used for both markets thus accelerating time-to-market and reducing development/manufacturing/distribution costs versus making a "world version" and a "NAM" version per usual. Assuming there are no contracts/agreements to the contrary, I would imagine that a proper "NAM" version (850/1900) of the N900 or its follow-on is in the works pending this "testing of the waters on the cheap".

just my 2 cents...I might be completely off base on this.