This is the method that has worked for us—if you know of any shortcuts or another way to enable MMS, let us know in the comments.
What You'll Need
• iPhone 3G running OS 3.0
• Another AT&T phone (non-iPhone)
• This carrier file
• SIM ejector tool (or paperclip)
Prep Your iPhone
1. Download the carrier file.
2. Connect your iPhone to your computer. Open iTunes, and option or ctrl click (depending on whether you're Mac or PC) "check for upgrade" which will open up your file browser. Select the .ipcc file. This is just a carrier file, it doesn't do anything to your OS or data. Uploading it to your phone will just take a couple of seconds. You're done.
3. Take your SIM out of your iPhone and pop it into a phone that's capable of MMS. Make sure it's registered to your account on AT&T's site (go into your account manager), replacing the iPhone as your listed phone. You might have to do this manually.
4. Go here, to AT&T's MediaNet site, and register your phone number. You'll get an SMS with a 4-digit code to confirm.
5. Put your SIM card back in your iPhone. MMS should be working now! It's kind of pokey (about 30 seconds to send a picture) and it's dropped a message or two, but we were able to send pictures to people on multiple carriers and receive video from a phone on T-Mobile. It definitely works, and works pretty well. The interface for playing back MMS video (pictured up top) is slick, too.
6. Wonder how you ever did without it.
Word of Caution
We're rocking an unlimited family messaging plan, so we're not worried about any charges for blasting out tons of MMS messages. If you're worried about account weirdness, you might wanna hold off. You could always ask your local AT&T guy to make sure you're not getting hosed, just tell him you're a developer (you've got iPhone 3.0, after all). Also if you're squeamish about unofficial OS files, obviously.
Big thanks to Scratic, who showed this method to us. If you know a better way, let us know!