[Sangetsu]

When the Iphone was introduced in America, you also needed a data plan, as well as a 2 year contract.

Japan's Iphone does not come with a removable sim card, which means using it with another carrier besides Softbank will not be possible (at least for the moment).

Your best bet is to get an American AT&T 8525 or "Tilt". This phone is unlockable, and you can use a Softbank sim card in it. It has many more features than the Iphone, though it is somewhat more expensive. Softbank sells the same phone in Japan under a different name, but Japanese versions are not unlockable.