According to media reports NTT have already shut down their PDC network in March. Now there's only their 3G network left.
However I've found this on the
Japanese Wikipedia:
Quote:
対応機種（海外端末） [編集]
W-CDMA#周波数帯に記載がある通り、UMTS850（バンドV）はUMTS800（バンドVI）を内包し ており、機器によってはFOMAプラスエリアでの通信ができるものもある。
例えば、iPhone 3GS、iPhone 4 はUMTS2100とUMTS850で通信できるため、FOMAプラスエリアで使用が可能である（ただし、 SIMロックがされていないことが前提である）。
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which Google translates into:
Quote:
Compatible models (international terminal) [edit]
W-CDMA # as there is in the band, UMTS850 (Band V) is UMTS800 (band VI) has been encapsulated by the device is also able to communicate with FOMA Plus-Area.
For example, iPhone 3GS, iPhone 4 because you can communicate with UMTS850 UMTS2100, it can be used in the FOMA Plus-Area (However, it is assumed that the SIM is not locked).
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This proves my old theory, that UMTS850 phones (which are very common in the US) are fully compatible with UMTS800 networks, which came to my mind after I saw, that UMTS800 not only uses only frequencies from within the UMTS850-spectrum, but also UMTS800 channel numbers (UARFCN) are within the range of UMTS850 channel numbers:
UMTS frequency bands - Wikipedia, the free encyclopedia
So UMTS800 just seems to be a frequency-wise cropped version of UMTS850, allthough the designation of these standards (800 vs. 850) suggests, that UMTS800 operates at lower frequencies than UMTS850.
So if you own a UMTS850-compatible Nexus One, you could use most of NTT DoCoMo's network including the "FOMA Plus"-covered areas (FOMAプラスエリア). Note that there are two versions of the Nexus One, of which the first one would suit best for Japan:
for AT&T customers: UMTS 850/1900/2100
for T-Mobile customers: UMTS 900/AWS/2100