Gavin Newsom's got a plan: He's going to stop those damn district-elected progressives from appoining a new mayor even if it takes some wacky legal footwork. According to the Chron's Matier and Ross: 
For the past two weeks, Newsom's political team has been combing the state Constitution to determine if the mayor, assuming he's elected statewide, could legally push back his Jan. 3 swearing-in for the new job until after Jan. 8.
If he can, the job of naming his successor would go to the newly elected Board of Supervisors, which is sworn in Jan. 8, instead of the current lineup.
I don't know where that team is looking in the state Constitution, but the language seems pretty clear to me. Article V, section 2, provides that the "Governor shall be elected every fourth year...and hold office from the Monday after January 1...." In 2011, that's Jan. 3. It also says (article V, section 11) that "The Lieutenant Governor, Attorney General, Controller, Secretary of State, and Treasurer shall be elected at the same time and places and for the same term as the Governor."
And since the mayor of San Francisco is, by Charter, a full-time job, Newsom can't be both mayor and lt. governor. Which means, I think, that he's got to start the new job Jan. 3, and the new Board of Supervisors doesn't take office until a week later.
There's another twist here: The City Charter discusses a "vacancy" in the office of mayor, and authorizes the Board of Supervisors to select someone to fill the remainder of a vacant term. If Newsom wins in November, it will be clear that a vacancy is looming -- and there's no reason why the supervisors can't pass a motion right away designating the person who they intend to have fill that vacancy. In other words, this current board could select the next mayor even before Newsom officially resigns.
Now, it's also true that the motion wouldn't become effective until the mayor actually left office, and could be rescinded at any time up until that moment. But if the supervisors find six votes for a candidate, and designate that person as Newsom's successor, it's unlikely the board would decide to change its mind and rescind in just a few weeks.
And even if all that doesn't fly, there's a very good chance that progressives will still control the next board. Four progressive supes will carry over -- Ross Mirkarimi, John Avalos, Eric Mar and David Campos. If progressive candidates win two of the three swing races -- in districts 6, 8 and 10 -- then the overall politics of the board won't change dramatically.
So there's actually a chance that a progressive mayor could take office next January. Whether Newsom likes it or not