Using your iPhone in Japan.
Occasionally I get asked can I use my iPhone in Japan.
The answer is of course you can!
The ironic thing is your iPhone will work better in Japan because you will be able to roam and use docomo's network, which is far superior to the crappy service that Softbank provide.
Now if you have an unlocked iPhone, things get a bit more interesting.
You can go to either Softbank or docomo store and ask to get a sim card for the iPhone. These are different to Japanese sim cards, not physically but some hardware way.
Subscribing to Softbank is pretty easy, you get a sim card, choose the white plan and data plan (there is only one data plan), then decide if you want the additional services. For what it's worth - I have the full enchilada and the iPhone costs me around ¥10000/m it has only gone well over that when I sent a ton of SMS messages to international destinations (won't be doing that again).
You could choose docomo (I just got off the phone to them). They are less than forthcoming with information on using an unlocked phone on their network. First of all - it depends on the shop. The lass told me to go to an official docomo shop and have them ring the international service desk to get the questions answered. She also told me some services might not work, or if they do work I would be charged higher rates for it. Secretly I suspect you could buy an android or blackberry phone, subscribe to the data service, take the sim card out and slip it into you iPhone and bingo you are up and running - it is an experiment I would like to try.
I don't know what docomo would charge for the sim card but Softbank charge just shy of ¥3000.
Hope this answers your questions.
