There may be developments sooner than you think.
Most political pundits expect a Cabinet reshuffle soon, and it's pretty clear that Greening will go, not least because she lost much of her remaining credibility when the Chancellor trampled all over her with his decision to freeze the fuel duty hike.
He, of course, has become a more public advocate of a third runway and, given a more amenable Transport Secretary, I wouldn't rule out some sort of commitment/U-turn before the next election.