Japan for voice?

[inquisitor]

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:

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）。

which Google translates into:

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).

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