Like Tim Montgomerie and Mike Smithson, I suspect that Gordon Brown and Nick Clegg will be happiest with the news. Both of them, particularly Brown, need potential game-changing events like this to make some progress in the polls.
As for Cameron, he'd probably be better off not giving his opponents a chance to make inroads into the Tories' poll lead. But he could hardly have resisted live debates in the current political climate. CCHQ will console themselves with the fact that their leader is an early favourite to come out on top.