The AT&T iPhone update to enable MMS just hit for us, so check your iTunes to see if you have it too. Just plug in your iPhone and hit the "Check for Update" button.
Many people are saying that the update runs fine, but they don't get the MMS button right after they update. To enable it, you'll have to reboot, so do that now. It would have been nice if the update automatically rebooted your phone, to save a lot of people confusion, but at least MMS is here! [Thanks tipster!]
Here's what an incoming MMS looks like.