I highly recommend reading Hatching Twitter (http://www.amazon.com/Hatching-Twitter-Story-Friendship-Betr...) which talks about the behind the scenes CEO transitions between first Jack Dorsey, then Ev Williams, then Dick Costolo. Both Jack and Ev were said to be forced out by the board against their will (per the book) - event though the press spin was that they left voluntarily.

I'm hoping that Nick Bilton writes a follow-up with this new Costolo to Dorsey transition. I wouldn't be surprised if Jack ends up removing the interim title and stays full time.