In a few days, if all goes according to plan, AT&T will push out a software update which will enable the iPhone 3G and iPhone 3GS to support MMS, but if you want to enable MMS on the iPhone on iPhone 3.1 right now, here’s how.
According to an article on quickpwn, you enable iPhone MMS by…closing iTunes if it is open, then go to applications>Utilities and open Terminal app, copy and paste defaults write com.apple.iTunes carrier-testing -bool TRUE into Terminal, return to keyboard and download IPCC file and save as ATT_US.ipcc. Next, connect iPhone to PC and open iTunes, press Optionon your keyboard and at the same time update in iTunes window, locate the ATT_US.ippc file and select it and then reboot and hold down the top button of your iPhone until the “slide to turn off” message appears, slide and turn your iPhone off. Turn your iPhone on again and it should have MMS on it!
If you use Windows there is also a way of enabling iPhone MMS if you hit up the link and follow the instructions.