GOOD NEWS, everyone! As of one hour ago, the option to send/receive MMS messages on iOS 6 has been restored! No need to unlock your phone or buy a T-Mobile SIM card! Finally, after 9 months, it's back! I'm using a prepaid iPhone 4 on an AT&T GoPhone plan. I can't guarantee this with other carriers as I just found out an hour ago.
The way I found out was when a friend of mine sent me a message containing a picture. I thought it was weird that she was able to send me a picture because she doesn't have an iPhone or any other type of iOS device, and the only time I could ever receive MMS messages was when I was connected to Wi-Fi, which I was not. I had almost full bars of 3G, and voila - the option is back!
I sent a test picture message to myself and thought it was odd that I sent and received it using iMessage. I then sent what I thought would be a text message to a few of my other iPhone friends, and what do you know, they also sent as iMessages! I could ONLY do this using Wi-Fi before. Neither of us had a Wi-Fi connection, so I suspect the option is fully restored! No more switching between green and blue bubbles when sending text messages!
I can't figure out what the fix may have been and why it took so long. My guess would be that AT&T has "officially" started supporting the use of iPhones on their prepaid plans. I think AT&T and a few other carriers killed this option during the iOS 6 development. Whatever the case is, it's fixed and that's all that matters.
Thanks a ton, AT&T and Apple!