The official word from AT&T is that iPhone MMS will arrive "late morning" Pacific time, or early afternoon Eastern time. You'll have to plug into a computer and hit up iTunes to get the update, but it's not clear exactly when MMS will turn on, so just have to keep refreshing until you get the go-ahead.
AT&T's customer care center has a recorded announcement up informing people that there's MMS latency issues around the country and that while the issue is being worked on, there's no estimated time for an actual fix.