defaults write com.apple.iTunes carrier-testing -bool true

Once you’ve done that go into restore mode in iTunes (option-click the Restore button) and choose the ipcc file you just downloaded.

Alternatively you can just click on this link from your iPhone to run through a quick generator to enable tethering. Doing so will generate a property list file with a .mobileconfig extension, similar to what you create in the iPhone Configuration Utility (if you’re like me you’ll want to see what this thing is doing before you cut it loose). Because the file is compatible with the iPhone Configuration Utility, you can actually download it onto your computer and double-click on it to add it into the iPhone Configuration Utility library and see which keys the payload will install. You can also open with your favorite plist-friendly editor and view the keys directly.