I bought my N900 from finland before coming to japan, im finnish so all i can use is nokia=)
Got it for like 700USD. I think the best GPS maps for japan is google maps, im sure the app will
be ported sum day? But yeah, without a fixed data plan it's a no go. Well anyway the prepaids are
ridiculously expensive in japan, so afaik. a big time contract is forced upon those staying for a longer time. 4000yen/month for unlimited free internet is not so bad.