Gideon may have to pull his finger out and make a decision, but Milliband is playing a very clever game at the moment.
As we know, the last Labour government actually approved a third runway, but the current Edd & Edd iteration has given no indication where it stands.
The significance of course is that if it were to support a third runway (as is the logical thing to do) then Gideon wouldnt need to worry about Clegg and the Liberal loonies think. However, if they are to go it alone then the Lib Dems opinion actually counts, as the Tories need their support in the coalition.
My suspicion is that Labour would support the runway, but will do everything in their power to avoid saying that right up until election time for fear of handing the upper hand to the Tories.