iPhone purchased outside Japan, usable with Softbank?

Going to Japan in 6 months. Just came back from there, so I know how a little about phone plans and stuff (I was with AU). I know Softbank is the only plan that the iPhone is compatible with, but would Softbank allow the use of an iPhone purchased outside Japan on one of their plans?

I was able to pop a softbank sim card into my unlocked iphone and use it pretty much seamlessly.

However, the sim card was from a candybar phone that had no data plan, and softbank will absolutely rape you on data usage with the iphone. I barely used it at all, but they said I used 10mb (part of which might have been passive data usage, I checked my email and facebook a handful of times and looked up one website to confirm the location of a bus stop). The grand total was close to $200 US.

So if you do want to use your iphone, make sure your plan includes data, or turn data off altogether.