I live in Japan and am on my third iPhone 3G and I also have a non-3G US iPhone for when I travel in the US.
Even unlocked it will be a paperweight because you will need to get a Softbank iPhone specific SIM in order for it to work. The only way to get that SIM is to sign a contract for an iPhone when you buy one in Japan. The color of the SIM is also different silver is the standard color for Softbank SIMs, the iPhone 3G SIM is black.
The SIM that Softbank uses in the iPhone in Japan is NOT compatible with any other handset offered by Softbank they are not interchangeable in either direction. This is intentional and they tell you this when you sign the contract (in Japanese of course) as a disclaimer.
So this essentially means you can not buy a non-iPhone Softbank phone with the intention of pulling its SIM out and putting it in your unlocked iPhone 3G. While, a Softbank iPhone SIM would probably work, you won't be able to get that without buying a Softbank iPhone thus defeating the point.
Other Softbank SIMs simply won't work as the provision information is completely different for the Japanese handsets. They also changed the moble email address of people switching to the iPhone to @i.softbank.jp to further delineate the two and I presume for assisted bragging rights and self-promotion (the iPhone is very much a fashion item here and I have seen lines even in the country side).
In fact, when I needed to replace my iPhone they had to locate one and I had to drive a good distance into the middle of nowehere to get it. Incidentally, they replaced it with a phone from a special inventory set aside specifically for replacements due to the policy about the SIMs not working on any other Softbank phone. In theory this would leave you phoneless in the even that your iPhone failed because you cant simply put your SIM in another phone and this would be completely unacceptable in Japan.
So they inventoried factory new phones that are not in the nice packaging but rather a brown shipping box with foam inside and nothing but a new iphone in it with no wires or anything else. They simply switched mine, no paperwork, no questions asked.
But roaming will definitely work as someone from Switzerland visited me recently and their phone worked flawlessly, albeit at the cost of two arms and a leg.
