Sorry, AT&T, but you can’t hide tethering from the very folks who taught you, long ago, what it meant to truly tether. First, download this carrier update and then type this into Terminal.

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

Finally, start up iTunes and option-click the Restore button and select the ipcc file in the disk image. Once the update is complete you’ll see Internet Tethering under your network settings. Easy peasy.

from MacMegaSite

UPDATE – Just go here with your iPhone browser. This enables tethering and if you reboot you can get MMS. MMS is still wonky on AT&T, though.