twesterm on 17 November 2008
Unfortunately I don't have a link for this, but apparently L4D is releasing today (11/17) and will be available to play on Steam at least at 11/17 11PM CST.
Preloading for the game started last week so get preloading so you can play right at 11!
Going to be a long night tonight. 
-edit-
This is about the best I can do for proof at the moment:
http://store.steampowered.com/app/500/
