IPad – Personal Hotspot for iPad 2 3G

It is not a carrier localized version of the OS that is installed on the iPhone. There is one iOS release image for each hardware model. What changes for each mobile network is the carrier settings, which is downloaded either OTA (iOS4+) or by iTunes when the phone is activated or connected and an update is available.

If you have the released from any carrier lock then if you move to another mobile provider supporting the tethering feature then their carrier settings will enable the preference.

There is no built-in logic to specify an if/then decision to not join WiFi so the mac will join any WiFi SSSID that is detected and matches its list of auto-join networks.

You can set the service order by "secondary clicking" the gear at the bottom of the Network Preference Panes to Set Service Order... and make sure the WiFi is lower than the USB connection. It will still join, but no traffic will flow while the USB connection is viable.

Lastly, you could just make a new Network location called USB Only and turn off WiFi all together when you select that location on the mac. Changing network locations can be scripted as a service and AppleScript/Automater or FastScripts to effect or reverse the change you desire from a keyboard shortcut.