"It was important to give our customers a positive experience from day one," AT&T said. "We support more iPhone customers than any other carrier in the world so we took the time necessary to make sure our network is ready to handle what we expect will be a record volume of MMS traffic. We truly appreciate our customers’ patience and hope they'll understand our desire to get it right from the start."
According to AppleInsider.com, the new feature will come in the form of iTunes downloadable update for 3G and 3GS owners. Unfortunately, owners of the original iPhone will be left out in the cold as they will be locked out from getting the update. I guess that is a subtle hint from AT&T that you need to upgrade.