While we wait, The Indy lists the possible scenarios:
David Cameron becomes prime minister. Gordon Brown moves out of Downing Street and gives notice he will quit as leader of the Labour Party on 26 September, the opening day of Labour's annual conference in Manchester. Meanwhile, he has to face Cameron every Wednesday at Prime Minister's Questions, during which he is ritually humiliated. Labour's poll ratings slip as it spends four months engrossed in a leadership contest, with the ever-present risk that the Tories might call a snap election. Once the new leader is in place, Labour's ratings improve.
Cameron becomes prime minister. Brown resigns as Labour leader with immediate effect. Harriet Harman takes over as temporary leader and, finding that she rather enjoys being in charge, persuades the National Executive that it can save a bit of money by delaying the leadership election until September. This gives her time to weigh up whether she should join the contest. Her weekly jousts with Cameron at Prime Minister's Questions are hit and miss. Labour's ratings fall until the leadership question is settled.
Cameron becomes prime minister. Brown resigns, Harman takes over, but the National Executive insists it cannot risk a leadership contest overlapping an election should Cameron go back to the country in the autumn. It decides on a one-day conference in July when the results of a postal ballot can be announced, and the new leader and deputy anointed. Two months of intensive campaigning follow. In July, Labour has a new leader, who spends the August break planning strategy for the autumn.
The comrades would be well advised to adopt Scenario 3, but Gordon Brown has to resign as Labour leader before the fun starts.