http://www.destructoid.com/valve-confirms-left-4-dead-dlc-will-be-free-f...

The word is April 21st, and free. The two "new" campaigns are already there in co-op/single player, I'm guessing they just finished off the stuff for making them versus playable. I haven't tried any vs play yet, so I don't really know much about it.

I'd take this as pretty good evidence that Valve will likely continue to update l4d like they do tf2.