1) It's not 1700/2100, just "UMTS1700", "UMTS Band IV", or more often "AWS". Yes, it uses some 2100MHz frequency, but calling it 1700/2100 is improper, misleading, confusing and incorrect per the GSMA and FCC. AWS is not compatible with UMTS Band I @ 2100MHz. It's like call PCS/UMTS1900 1800/1900, which suggests it's compatible with UMTS 1800, which it also is not. http://en.wikipedia.org/wiki/UMTS_frequency_bands
2) AWS is used in the Americas. T-Mobile was just the first to use not, but not the only one.
3) A version of the Dell Streak passed through the FCC with UMTS1700 support. SRC: http://androidandme.com/2010/02/news...-aka-t-mobile/