The guy behind StealthBravo has managed to modify the latest AT&T Carrier Update file v6.0 so that it enables MMS and Tethering functionality on iPhone firmware 3.0 or 3.0.1. This will only work for AT&T customers with iPhone 3GS/3G running firmware 3.0.1/3.0.
Simply download the latest AT&T carrier update (.ipcc) file version 6.0 and follow the step-by-step instructions posted here from Step 1. Make sure you use the latest .ipcc file (download link given below) when prompted to download in Step 3 of the instructions posted here.
Images Credit: PimpvilleDeli
Ask me any question should you run into problem activating MMS or Tethering on your iPhone running firmware 3.0/3.0.1.
Download AT&T Carrier Update version 6.0 (.ipcc file)