We can argue this all day, I'm convinced Apple is going to wait for AT&T to release 3.1 to avoid 2 updates so close to one another, but we won't know anything until 3.1 is released so its pointless.
We may never know. If MMS is turned on by AT&T coincident to 3.1 release, we won't know whether AT&T waited for Apple, or Apple waited for AT&T.