iPhone tethering via USB not working

Solution 1:

It seems the Network prefs window is one of few settings windows where you have to Apply your changes before they take effect. I clicked on Apply and now it works.

Solution 2:

The 64 bit kernel apparently works since 10.6.3.

Also, I found this thread on Apple's support forums says to try and USB tether with iTunes running.

I had my old iPhone working fine via usb, but it stopped when i got a warranty replacement. You have to remove all the old iphone devices if they're in your System preferences, turn on iTunes, and then plug in the phone to find tethering.

Up until I tried to start USB tethering without running iTunes (i don't automatically sync with itunes when I plug in the iphone), I just got a message that said the device hadn't been setup, and when I'd go to setup the device, it would say the device isn't connected.

Hope that helps!