Left 4 Dead is a game that is not being supported with new DLC which is why people are angry. With that in mind, Left 4 Dead's new DLC support appears to be heading our way a little later this month, with Valve Tweeting that September 23 has been penciled in as the release date.
Valve's news comes via Xbox.com, which is advertising one of Microsoft's "Game with Fame" promotions where players can go up against a band I've never heard of, Paramore, in a round of Crash Course on September 23. So, even though Valve says that there's "no guarantee" that this is the real date, Microsoft seems rather certain.
Having played Crash Course, I can confirm that it's a very good campaign and well worth checking out. Of course, those with Xbox Live have to bloody pay for the thing (even though we're already subscribing to Live), so maybe this is only news that PC gamers can get truly excited for.
[Reader Bluewolf72 points out that the Valve Twitter feed is an unofficial account. They merely via'd the news from Xbox Live. Thanks for heads up!]