I have stock firmware with no jailbreaking but I do have the 5.0 ipcc hack. The ipcc hack is necessary or else the MMS option in Settings won't show up. However, I'm still running OS 3.0. What is absolutely necessary is that AT&T updates your account on their end so you won't be charged per MMS.
Originally Posted by Justin
Each account has to have the MMS provisioning turned on and the iPhone text message plan must be changed to a regular multimedia messaging plan. These are the measures that AT&T put in place so that if you temporarily succeeded in getting MMS working, you wouldn't be able to continue to send and receive MMS due to these steps taken to secure the backdoor, so to speak.
I've had my MMS working smoothly and flawlessly after speaking with an AT&T rep who removed these security measures way back in March/April. No deception was necessary with them at that time as I explained I was beta testing.
Another thing that I did was place my SIM card in my friend's MMS capable AT&T phone, send a few picture messages, make a few calls and later I placed it back in my iPhone and rebooted it. I believe this is also necessary in order to rewrite the SIM card to configure it for MMS capability as well. I can imagine this taking quite some time to do on millions of iPhone accounts on AT&T's network!