Well, cellphone companies are really strict in Japan.
If you go to the mobile companies shop with your unlocked iphone they won't sign you up and make you enter into a 2 year contract and pay up front for a phone at their kiosk.
There is a way around it though. Before you leave, research the mobile company you want in Japan. Then when you get to japan go buy a used phone from that company at a recycle store. (completely legit and legal)
Go to the mobile phone company and say you want to get the service you want and that you have a phone that you bought from a recycle store.
Once you sign up, toss the sim card in your iphone, input the apn info from your provider (If you get data) and voilà!
As for contract and prices I'm not too sure. You can really customize your service. I think you could check on their websites if you're up for a nihongo challenge though.