I've got the Internet Tethering showing in the 2g. I also got the connection Apple adapter in Windows, but says network cable is unplugged.

It seems to be controlled by /usr/libexec/misd (Mobile Internet Share).
Modifying the Library/Preferences/com.apple.MobileInternetSharing.plist State entry to 1023 (as per h**p://www.iget.dk/?p=234) seems to add the toggle, but something is missing.

I did add the tethering info to my carriers.plist, but maybe is wrong and hence the network cable unplugged error.

If you cannot get the toggle within networks, kill the misd and try again. I will keep playing with it and see if I can make it works.