Damn near impossible to unlock Softbank! The only outside chance you have is if your iPhone is 8GB. The 16's and 32's are completely shut out.
Even then, it is expensive and takes two weeks, minimum.
I know... I picked up a fairly cheap iPhone right here on MacRumors that was Softbank locked and went all around looking for a breakthrough.
I did order a Softbank SIM off of eBay so I could at least use it as an iPod. I did so until I finally was able to sell it during the Christmas 30% bonus promotion GameStop was running a few weeks back.
I don't miss that iPhone nor the experience!