I believe the iPhone 3G/3GS is the exact same model whether you buy it in Japan or N.America.
I brought my Rogers iphone 3G to Japan and used it seamlessly via a roaming agreement Rogers had with Softbank. (Mind you, it cost around 100 yen/minute but that's a different story.)
If you get an unlocked 3G, I'd imagine you'd have no problems using it with a rental SIMM from Japan.
With that being said, $235/2 weeks is pretty expensive! At least you can use Skype to call if you have an unlocked phone.
