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In this tutorial, people have reported success enabling MMS on iPhone OS 3.1 using either iTunes 8.2.1 or even iTunes 9. The process does not require you jailbreak your iPhone. It does require downloading a modified bundle file ,  the process seems easy enough and most are having success.

Welcome to the newest tutorial on how to get MMS working on your AT&T iPhone! Keep in mind this tutorial is for users of iPhones running iPhone OS 3.1 on AT&T’s network. The settings provided here may or may not work with earlier versions of the iPhone OS, and certainly will not work on any other network.
I believe the issue with you still getting the exclamation is due to AT&T’s ongoing process of removing the opt-out codes. Slowly but surely everyone will be given access. Keep trying periodically over the next few days.

The first few steps are the same as before, we must enable IPCC updates in iTunes 8.2 (9.0 works too). Here are the instructions from my previous walkthrough, nothing has changed except the IPCC file you download.
Mac OS X:

1. Close iTunes if it is open
2. Start Terminal (Applications > Utilities)
3. Run the following command: defaults write com.apple.iTunes carrier-testing -bool TRUE

Windows 2000/XP/Vista (32-bit):

1. Go to Start -> Run (
2. type in “command” and hit enter
3. Type/copy+paste this into the command console that comes up: “C:\Program Files\iTunes\iTunes.exe” /setPrefInt carrier-testing 1
4. Hit enter

Windows XP/Vista (64-bit):

1. Go to Start -> Run (
2. type in “command” and hit enter
3. Type/copy+paste this into the command console that comes up: “C:\Program Files (x86)\iTunes\iTunes.exe” /setPrefInt carrier-testing 1
4. Hit enter

Download the modified bundle right (right-click and “Save as” or whatever your browser’s equivalent is). To install this, follow these steps.

1. Download the file to some location on your computer
2. Plug in your iPhone
3. Start iTunes
4. Click on your iPhone in the device listing in iTunes
5. Alt-Click the “Update” button (Shift-click in Windows)
6. Browse to where you saved the IPCC you downloaded
7. Select this file and click Choose (or OK on windows i think)
8. Reboot your iPhone