As the AT&T iPhone MMS launch is only one day away now, Engadget is reporting that they heard some launch details from AT&T. Specifically, AT&T has stated that it expects MMS for the iPhone to launch in the "late morning" Pacific time, which would mean it "early afternoon" Eastern time.
Also, this update will arrive as a carrier settings update, which means you'll have to connect your iPhone to iTunes to update. These updates are very small and don't require a restart. Just make sure you and your iPhone are ready to update on Friday.