Our iPhone 3GS turned out to be cheaper than we thought. Not only did we pay nothing for it (we do pay about 6000 yen for unlimited net usage per month though), Softbank also sent us 20000 yen worth of gift vouchers than can be used at most retail stores across Japan. *And* we get to use all Yahoo mobile hotspots for free. The best thing is that most of the people I know use Softbank meaning that calls from 1AM through to 9PM are free.
If you are moving to Japan then I recommend Softbank. Tried AU and Docomo previously. Docomo was expensive while AU had crap phones.
Japan Mobile
Folks in Japan who recently got an iPhone 3GS should have filled out some form online to get the vouchers - instructions came with the phone.