Without wishing to get too political, the Infrastructure Commission seems to have been set up for two main reasons. Firstly, to give George Osborne an excuse to poach Andrew Adonis from the Labour Party; secondly, to create one body which will drive through major national instrastructure projects.
If a decision is to be made on R3 (a big if), I wouldn't be surprised if Cameron and Osborne announce that they will devolve responsbility for the decision to the Infrastructure Commission and back the result, as a means of limiting their political involvement with it and ducking the 2009/10 pledges. My understanding is that nearly everyone at the top of government (who hasn't a London seat) now sees that Heathrow expansion is in the national interest; the Commission may provide the excuse to back it again.