Having worked for a cell phone manufacturer, I can tell you there are a LOT of factors that go into bringing a phone and/or adoption rate of the marketplace.
The hard truth is - the United States is a good 3-5 years behind countries such as china and japan for technology adoption.
Obviously we know the technology exists. But user adoption (and I'm not talking people who sign in here who are obviously more technologically advance than a lot) is slower here.
Asian countries were texting with great speed for several years before texting even became popular here. And that has to do with the fact that they were more savvy at picking up t9 typing. American's were much slower in not only adopting the idea, but using it on a regular basis to warrant a need/demand.
EMS (enhanced messaging) with pictures and midi ringtones were the next step with MMS planned for 1-2 years after EMS. The adoption rate by the mass public is again, much slower.
It's a vicious cycle - many people would love the technology, but the initial demand/usage doesn't yet (to the carriers) warrant the cost of upgrades while also figuring out how to charge for it appropriately to make money.
Take a poll amongst the "normal" American cell phone users if they know what MMS is (without explaining any context, or setting the question up) and you can bet most have no clue.
Any product and company desires a saturation point. That involves planning, marketing and education of the consumer.
Look at high-def TV's and its adoption rate. And would you believe the amount of people that required a box when the stations switched to all digital?
Point is - all these features will come. They will be rolled out in a very planned and organized manner. And it's not just ATT. It's across all the providers.