Starting at 10AM Eastern (on the 25), AT&T will send out a mass text to a group of iPhone users telling them that MMS now works on their phone... They will keep doing groups of phones on the hour throughout the day.
AT&T spokesperson Seth Bloom is a little more cautious on the launch time frame, saying "We are targeting early afternoon Eastern time."
DSLReports notes that AT&T is very nervous about the MMS launch estimating that network traffic could spike by as much as 40 percent tomorrow as iPhone customers begin to send pictures and videos to each other en mass. AT&T is already seeing "record traffic during peak hours of the night" from users testing the MMS feature. The report also notes that AT&T is requesting hourly updates on message delays or problems from its MMS aggregator partners.
AT&T notes that "MMS will be enabled through a software update" -- so keep an eye on your Messages application and post a TalkBack when you get your MMS notification.