It's actually a little more complicated than this and the terminology makes it very confusing. The key to this is that the frequency band is different from the actual frequencies used for the radio.

Much of the world's 3G usage is on the 2100 frequency band. This actually operates on two frequency ranges, ~1900 uplink and ~2100 downlink.

In this same vein, AT&T has 2 separate frequency bands, 850 and 1900. 850 operates on ~824/~869 and 1900 operates on ~1850/~1930.

T-Mo actually only has one 3G frequency band, 1700 MHz. This operates on ~1710/~2100, but actually this is not compatible with the world-wide 2100 frequency band.

Most, if not all, 3G T-Mo USA phones support at least the 1700 and 2100 frequency bands. Nexus One also supports 900 MHz, which makes sense since Vodafone uses that band in Europe and Nexus One will soon be on Vodafone.

See http://en.wikipedia.org/wiki/UMTS_frequency_bands for more exact details.