Yeah thats right!, It took me too long to realize that, but its true.
You CAN NOT do usb tethering with a 64Bit Snow Leopard Kernel, I guess the drivers are still 32Bits.
You will not find this info on apple because, first tethering is not available on many countries (yes I have a jailbroken iPhone I’m not ashamed of that), and second the 64bit kernel is still targeted to servers, not users.
But I guess you are a power user 
, so don’t waste more time trying via usb, and do the tether stuff with bluetooth, it works great
.