what kind of mobile phone do you have?
i got a nokia N95 before i flew out to japan last year, and it has built in GPS. The good thing about it is that you dont need a local data connection as it will triangulate with the satellites itself with its gps antenna.
of course it is faster if you let the phone use a data connection, but i've found it unnecessary,... i'm not really ever in a rush anywhere. but yeah as long as you have clear sky above you, not blocked by too many buildings, it does its thing, and it does it quite well. it also has a wifi connection, so if you dont want to pay for data, but you are at a free hotspot, you can connect to it and then it will triangulate your position using the wifi,.... free of charge.
nokia maps has a japan and tokyo section, and you just download it to your pc and upload it to your phone, and youre good to go. its in english, but some of the more obscure places might be in japanese.
its worth a shot to check out.
now a days, if your phone is 3G or above, it will work in japan, but of course you wind up paying alot more for connecting calls, but its good for emergencies etc.
some links to check out:
Nokia N95 Trial: 30 Seconds Over Tokyo with Nokia Maps 2.0
Nokia Europe - On your mobile and online – Ovi Maps
ok theyre calling it ovimaps now, and to be honest i havent downloaded a map in about a year, which is ages in mobile phone terms, so there must be loads of new stuff around.