
Do this at your own risk. I haven't heard of any problems using this technique, but I am not responsible for any "hacking" you do to your own iPhone.
Download these carrier settings
Make sure there is no .zip extension on the second file after download. Sometimes Safari interprets this as a compressed file, so take it off. Sometimes you have to hit Apple-I to REALLY change it. The icon should not be a zip icon. It should be a blank icon.
Make sure your iTunes is closed, and open Terminal. Paste the following into Terminal without the quotes "defaults write com.apple.iTunes carrier-testing -bool TRUE"
Open iTunes and Option-Click on the "Check for updates" button. Select the Carrier file (ipcc file) and iTunes will send it to the phone. It won't give you any instruction, so just reset the phone when it's done. After that, go to your phone and select Settings>General>Network>Cellular Data Network> and enter these settings. Leave everything else blank...
APN: wap.cingular
Username: wap@cingulargprs.com
Password: CINGULAR1
MMSC: mmsc.cingular.com
MMS Proxy: 66.209.11.32
Under Network Settings you should now have the option to enable tethering. You should also have MMS working as well of your plan is set up for MMS. If not... good luck on the phone with them. You can install the current iTunes if you want and put your library back at any time.
NOTE: Sometimes the tethering options disappear. Select the IPCC file again in iTunes and it should reappear without restarting.