After his open criticism of David Cameron's cabinet reshuffle and his ill-disguised interest in an early Commons comeback, Boris Johnson's decision to establish his own review into airport capacity represents a further escalation of tensions with Downing Street. Unlike the government's inquiry, which he memorably dismissed as a "fudgerama", Boris's review will explicitly exclude the option of a third runway and is expected to take nine months, reporting two years earlier than Cameron's.
The line from the Mayor's camp is that the inquiry is not "a rival" to the government's, rather it is designed to "inform" it. But Boris's motive is clear. Aware that the government cannot break its pledge not to build a third runway before 2015, he will attempt to assemble a powerful coalition of support for his preferred option of a new airport in the Thames estuary ("Boris Island") on which work could begin immediately.
Meanwhile, the Mayor has unsurprisingly dismissed claims that Zac Goldsmith offered to stand down for him if the government breaks its Heathrow pledge (Goldsmith has repeatedly warned that he is prepared to trigger a by-election), stating: "I've said, as I said in the election about a billion times, being mayor is the best job in British politics and it's what I want to do."
But, as you will have noticed, Boris's choice of words doesn't explicitly preclude the possibility of his standing in a by-election. It's worth remembering that after the 2000 London mayoral election, Ken Livingstone remained the MP for Brent East until 2001. There is no constitutional obstacle to Boris similarly combining the two roles.