In a way, the endorsement is not too surprising. Having called on Labour to depose Brown, it would be quite hard for the paper to then argue that its readers should vote for the party he still leads. There is also considerable overlap between the Guardian leader line and the Lib Dem manifesto.
The endorsement will not only help the Lib Dems at this election but in the post-election period. The danger for the Lib Dems post-election is--if the Tories have won a majority—that all the media attention is on the new government and the Labour leadership contest, squeezing out the Lib Dems. But if the Guardian editorial line is to remain pro Lib-Dem for a while, that will help Clegg get the post-election coverage he needs--The Guardian is probably the paper that influences the BBC most. (At the moment, the broadcasters are obliged to give the Lib Dems a certain amount of airtime because it is an election).