While Valve and EA have never divulged an actual launch date other than “November 2008” for L4D, Gabe Newell made it known on video at GC that the game would launch on the 10th anniversary of Half-Life.
“Left 4 Dead is going to be released on the tenth anniversary of Half-Life, our first game, so that will be on November 20th, the third week in November.”
We’re currently confirming this with EA and Valve.