SIM unlocking is expected in the next couple of years, according to the telecom ministry in Japan.
I thought iPhone use in the US is restricted to AT&T subscribers, meaning it's locked there, too? If so, I don't know whether iPhone will become unlocked when unlocking happens for Japanese cellphone makes.
iPhones in the US are locked so you can use them with AT&T only, but some people can 'jailbreak' them, making them usable with other carriers. I'm not sure what would stop people from unlocking iPhones in japan :?
I'm not very knowledgeable in that area though, so..