http://www.computerandvideogames.com/article.php?id=218770
Interesting interview
I'm not convinced. When you look at the custom stuff that came out before the SDK you see that a campaign can be made fairly quickly. They still haven't released a new one.
Survivor mode was chopping up other levels, it's a nice addition, but not the campaigns we were all expecting "soon after launch".
I'd imagine a new campaign will come out just after the L4D 2 launch (late December maybe) and that'll be it.