I called earlier this year when AT&T said they were going to roll it out. Talked to U.S. rep guy and hey said with out hesitation that MMS was not going to be supported or enabled for the 1st Gen iPhones. Weather or not that is a a technical truth or not. I would presume that the 1st gen phone possibly could handle MMS. I think that AT&T just doesn't want all the extra iPhone on the network if they can help it and are limiting MMS to the 3Gs only.
I have a 1st Gen iPhone and not pleased that marketing is making people that owns 2G phones, upgrade to 3G's if they want MMS. Greedy Corportate America at it's finest.