As we reported earlier, AT&T is definitely rolling out MMS this Friday.
From the horse's mouth, via their Facebook page:
MMS Update: We know you've been eager for this service so we wanted to offer a quick update on the launch plans for MMS on Friday, Sept. 25. Late morning, Pacific Time, on Friday, the new carrier settings update enabling MMS should be live and ready to download through iTunes. We'll provide the steps and all of the details you need right here at that time.