Can't think of any reason, other than they just haven't done it yet.
But - I did find the still gallery pics all are AT&T.
I think the transition is stepping up, though - I just got a somewhat confusing email from AT&T Wireless reminding me my billing statement is available. I first thought it was my AT&T Long Distance. Also, if I go to login using my Cingular account bookmark, it ends up at AT&T.