As someone who often buys both Nano's and ipod video's in Japan - I usually find the price is equivalent or after (electric shop)points discount and 5% tax refund a bit cheaper than USA - and I presume the ROW.

This is usually down to the on-going weakening of the Yen against $

There is a small electric shop in the airport - and I guess one of the electric chains in Narita town.

I bought an ipod 60GB video last month and I think (??? - I could be wrong) it came with a one year worldwide warranty - but they will be able to tell you when you buy it - and ipods often go wrong so worth being 100% sure about this part.