This evening's coverage has also made it clear that David's resignation from the Labour front bench was inevitable. The media fixation on the brothers' relationship would have made life insufferable for both of them and dominated the coverage of Ed's leadership if he had tried to stay there.
That said, there is a lot to be said for the arguments of The Third Estate (if not for its spelling). This site points out that most defeated candidates for the Labour leadership have stayed on the front bench afterwards because they have a political agenda they wanted to pursue.
It then asks what it is that makes David Miliband different:
It is certainly not politics: after all any disagreements he has with Ed, pale in comparison to the gulfs that seperated Bevan from Gaitskell, and Healy from Foot. The brotherly aspect of it may be a factor, although Ed and David have had decades to get used to combining familial commitment with the mesy business of politics (could things really get worse than when they were in the opposing Blair and Brown camps?).
What does differentiate David from most previous runners-up is that he is a career politican par excellence. He did not come up through the trade union movement, nor did he ever have to prove his fealty and commitment to constituency party party members. After taking a job early on with the IPPR, he became an advisor to downing street before being parachuted in to a safe constiuency – a now well worn path.
For him politics has been a fairly continous journey up a career ladder. In the same way that people in blue-chip firms tend to reach a point at which they go up or get out, David, it appears, feels the same about Labour right now.