Just to clarify, the modern DoCoMo phones are supporting three 3G bands - '800', '1700' and '2100'.
800 - The so-called Japanese 850 band, called 'FOMA Plus' by DoCoMo's marketing bunch. It's this band support that allows DoCoMo phones to work on 3G 850 networks outside Japan.
1700 - A band unique to Japan. This band was added to alleviate bandwidth congestion issues on the 2100 band in cities.
2100 - The regular 3G 2100 band, used in Japan and across the world.
For the record DoCoMo phones don't support 3G 1900. GSM wise they're typically supporting GSM 900/1800/1900.