Softbank (formally Vodafone) is Japans carrier for iPhone.
(its a pretty big world out there and there are a lot other countries where various providers have the iPhone on their network)
Tethering is working for you? Wow, I thought with Softbank's no tethering and locked sim this would be a no go.. I may have to investigate more.
However I would be interested to know how your reception is.
I tried phones at Yamada, Softbank and the Apple store in Tokyo, other than at apple the reception was poor and then completely unusable if I held the phone in my left hand, how are you finding it?