As you know, AT&T has officially released the MMS feature to iPhone users. However, as it said, you have to upgrade to iPhone OS 3.1 to use the feature. It’s not a problem for those not considering jailbreak. Just upgrade to iPhone 3.1 and apply the carrier update from AT&T to enable the MMS feature.
For those who are using jailbroken iPhone, however, you are probably staying with iPhone 3.0 and waiting for iPhone Dev team to release a jailbreak solution. Despite that, it shouldn’t stop you from using MMS feature on AT&T. Even AT&T claimed that you need to have iPhone 3.1 to use MMS, there is a hack to enable MMS feature on jailbroken iPhone 3G using firmware 3.0. [Thanks for savior1974 and krillr for providing the solution!]
Note that the solution is only for AT&T user with jailbroken iPhone 3G/3GS running on 3.0/3.0.1.
Before following the procedures in the guide, make sure you’re using iTunes 8.2 or 9.0. You can upgrade to iTunes 9.0 by downloading the free copy from Apple.
For Windows
1. Download this carrier update file (v5.5) from here (or here)
Note: If you need to enable MMS/Tethering/Visual Voicemail, please download the carrier update file v6.0.
2. Close iTunes
3. Click “Start” -> “Run”. Type “cmd” to launch command prompt.
4. In command prompt, type: (make sure you type the double-quote)
“C:\Program Files\iTunes\iTunes.exe” /setPrefInt carrier-testing 1

Note: If you’re using 64-bit Windows, type “C:\Program Files (x86)\iTunes\iTunes.exe” /setPrefInt carrier-testing 1 instead.
5. Connect your iPhone and launch iTunes
6. In iTunes, click on your iPhone. Then hold the SHIFT key and click the “Check for Update” button. This will bring up a dialog for you to select the carrier update file.
7. In the dialog, change the file type to “iPhone Carrier Configuration files” and switch to *.ipcc file type.

8. Locate the carrier update file you have downloaded in step 1. Select and open it.
9. Wait for the update. Once completed, restart your iPhone. The MMS feature will be enabled after reboot.
For Mac
1. Download this carrier update file (v5.5) from here (or here)
Note: If you need to enable MMS/Tethering/Visual Voicemail, please download the carrier update file v6.0.
2. Close iTunes.
3. Launch terminal application and type in “defaults write com.apple.iTunes carrier-testing -bool TRUE”

4. Connect your iPhone and launch iTunes
5. In iTunes, click on your iPhone. Then hold the Option key and click the “Check for Update” button. This will bring up a Finder dialog for you to select the carrier update file.
6. In the dialog, select the carrier update file you have just downloaded in step 1.

7. Wait for the update. Once completed, restart your iPhone. The MMS feature will be enabled after reboot.
Again, please note that this hack is for iPhone 3G/3GS user with firmware 3.0/3.0.1 only. For iPhone 2G user, you can install ActivateMMS2G via cydia to enable MMS feature.



