From:
Hack to Enable MMS on iPhone OS 3.0 - iPhone Hacks
Enabling IPCC Updates for iTunes 8.2:
iTunes 8.2 will not allow you to apply the IPCC updates so you need to follow these steps:
MAC OS X:
1. Close iTunes.
2. Open Terminal (Applications > Utilities > Terminal).
3. Run the command: defaults write com.apple.iTunes carrier-testing -bool TRUE.
Windows 32 Bit:
1. Close iTunes.
2. Go to Start then Run and type CMD.
3. Run this command(If you right-click on the cmd window you can paste): “C

Program Files\iTunes\iTunes.exe” /setPrefInt carrier-testing 1.
Windows 64 Bit:
1. Close iTunes.
2. Go to Start then Run and type CMD.
3. Run this command(If you right-click on the cmd window you can paste): “C

Program Files (x86)\iTunes\iTunes.exe” /setPrefInt carrier-testing 1.
Copying the IPCC file to your iPhone:
The above steps will allow you to update the IPCC using iTunes 8.2. To copy the IPCC file to your iPhone follow these instructions:
1. Open iTunes & plug in your phone.
2. Click on your phone in iTunes(under device settings).
3. Windows Users: hold shift and click “Check for updates”. MAC OS X: hold alt and click “Check for updates”.
4. Select the IPCC you downloaded earlier.
This is where your other AT&T phone comes in handy. You need to follow these steps to enable MMS on your iPhone:
1. Remove the sim cards from both the iPhone and your non-Apple phone
2. Place the iPhone sim card into the non-Apple phone
3. Power on the new phone
4. Make a phone call or two (your voicemail works)
5. Log in to your myWireless account at wireless.att.com
6. It should now say that your device is whatever you put your SIM into. If it doesn't, repeat step 4 a few times.
7. Go to the "Features" tab
8. Select a "Messaging Plan", one that isn't an iPhone plan.
9. Click next and go through all the legal crap
10. You should now have MMS enabled on your account.
11. Swap the iPhone sim card back into the iPhone.
12. Reboot the iPhone.
Note:
AT&T Family Plan users who have a family messaging plan can skip steps 7-11.
Please don't add an iPhone texting plan back to your account, as it will disable MMS again.
Aaron has reported that AT&T may be blocking these steps from immediately taking effect so he has adviced users to call AT&T after step 10/11 and before step 12 to complain that MMS isn't working. According to him this should fix the issue.