I called today to have it provisioned and had zero issue at all. It was actually quite opposite from what all the other posts on this forum indicated.
I simply said "Hello, my name is blah, I'm an iPhone developer and have recently upgraded to OS 3 beta 4- I love the new search feature btw- but I'm wondering if you could please provision my phone for the MMS functionality?"
The lady put me on hold for about 3 minutes while she spoke with her supervisor because she was unclear of how to handle that- came back and said it has been provisioned and that she changed my iPhone exclusive SMS plan to a blanket "Unlimited Messaging" plan, which includes MMS/IM/SMS/etc.- and she did- and to restart my iPhone for the changes to take effect. She did mention a $0.67 pro-rated fee for changing plans mid-service month, no biggie.
I restarted and she tried to send me a MMS and she had me try to send one, neither worked (this is with the updated .ipcc & being registered with MEdiaNet). She explained that she couldn't guarantee anything would work, but we tried again and it failed again. She tried to contact Apple directly for support and after 20 minutes on hold with them, then gave me the number to the Apple 3.0 Developer Support Center- but it was after 5pm and I decided not to even bother today, tomorrow I'll call them.
On an off topic note, even getting 'provisioned,' having the correct ATT_US.ipcc and registering for the MEdiaNet account, like I have, doesn't necessarily mean everything will work. Mine still won't send MMS's and when people send them to me, irrespective of carrier, I get no message at all, not even the oldschool "I sent you a multimedia message, go to viewmymessage.com/2 blah blah" message.
Your mileage may very, based on all the reports of members here and on other forums. Just be happy, speak with confidence, and be absolutely professional and courteous at all times. Maybe I got lucky.