If you are planning on using a cell phone during your stay, I suggest that you rent one when you arrive.
I don't know if this is the same for every country, but I have not seen any European cell phones work there.
If I remember correctly, you can rent cell phones at the airport. It is a little expensive (hey, almost everything there is)
but it can be more convenient, especially if you are going outside of Tokyo, because not all phones there accept the same calling-cards.
I found that the card I used for the public phones in Tokyo were useless when I arrived in Sendai.
And as another person pointed out, make sure to bring a lot of cash with you.
Japanese people normally prefer to have thick wallets full of cash rather than using credit cards.
You are lucky if you find an ATM there that accepts Master Card.
VISA is even less accepted.