Two of Japanese Cell phone carrier (DoCoMo and SoftBank) have standard 3G network and force their subscriber to use their USIM on their own bland handset for their cheap price plan (except for roaming).
You could use your N95 with Softbank USIM and you can ustilize Softbank rental service.
Mobile Phone Rental in Japan│SoftBank Global Rental (in English)
FYI:
NTT DOCOMO: HOME (English TopPage)
FOMA (in Japanese)
Discussions: (in English)
Phone Rental in Japan? - Boing Boing Gadgets
japan-guide.com forum - 3G phones compatible in Europe/Japan?
And another carrier e-Mobile has also 3G network but the frequency is not standard 2.1GHz (1.85G? I forgot). If you plan to make data communication, this could be another choice.
https://store.emobile.jp/customer/in...utlet_emcharge (They have no English Page)
EDIT:
The use of "Standard" is not correct. e-Mobile also based on standard 3G. However, they are assigned minor carrier frequency.