I tend to think that the polls would initially suggest that Labour would make gains on 2005, but that would evaporate over the course of the campaign and it would end up with Labour taking some losses and ending with a majority of say 25-45, depending on how disaster-prone Brown's campaign performance ends up being.
Cameron would stay on--it was always understood that he would be a two-election project and he'd make some gains of seats in places like Kent and Essex that barely hung on for Labour in 2005--but the Lib Dems would probably take some losses to the Tories and ditch Menzies Campbell.
Scotland would be fascinating, with both Labour and the Lib Dems led by Scots and the Lib Dems trying to defend some Iraq-driven 2005 gains that might snap back with Brown replacing Blair.
Brown would then hold on for the next 5 years as the financial crisis hit, survive several rather half-hearted coup attempts and cabinet resignations, and end up going down to the Tories in 2012 both out of fatigue and the economy remaining in dire straits. Hard to say how big or narrow a defeat it would be though.