Custom campaigns *drool* They made L4D unmodifiable, but I'm sure they will come up with some ways to add awesome stuff to it. *Is waiting for MetaMod:L4D*
Valve just want the SDK to be properly functioning, compared to the OB SDK which crashes if you do one thing it doesnt like.
Just found the teaser trailer for this campaign! http://www.wegame.com/watch/Left_4_Dead_Campaign_Dead_before_Dawn/