Hi all. First, this thread has been extremely helpful. I got MMS working on my iPhone. I followed what a few others discussed earlier.
I should note that I am on a family plan. I don't know if the standard iPhone text message plan will work.
First, I followed all the directions in terms of updating the carrier file. My settings have only the APN, mmsc, and mmsc proxy filled.
It wasn't working at first; I had the ability to take a picture in messages, but it wouldn't send. I removed the sim card from my iPhone and my mother's crapPhone and put my sim card in the crapPhone. After turning it on and off a couple times, I was able to access MEdia net and send an MMS. I then replaced the sim cards, and it was working on my iPhone.
So, if you are on a family plan and have another phone lying around, I am fairly confident this will work.