The ability to send and receive multimedia attachments via text messaging has been a long desired function of iPhone users. iPhone users were delighted when Apple released its new OS 3.0 for iPhone in June with MMS support. That joy was quickly faded when AT&T restricted the use of that function for fear of service quality issues with so many users sending data over their network.
AT&T promised that the function would be available in the Fall and appears to be making good on that promise when they announced the September release date.
It seems that some areas of the country are being switched on early as may be expected so as to have it all up and running by the 25th.
To see if your iPhone is MMS capable early, just give it a try and see if it's working yet.