One of the main successes of the first "Left 4 Dead" was the demo. With a new franchise it's important to inform the planet why it's awesome, and the demo did just that, giving folks a healthy taste of what "Left 4 Dead" was all about.
Valve is continuing the trend with "Left 4 Dead 2," with a demo going out to folks that pre-ordered the game on October 27th. Less committal humans that are holding off on pre-orders can get access to it on November 3rd (so long as you're on a PC or you're an Xbox Live Gold member). Come November 10th, the demo will open up to all members of Xbox Live (for those that enjoy a little split-screen multiplayer).
The developer also released information on what the demo would include, taking a segment of gameplay from the "Parish" campaign. An odd choice, since I'm pretty sure "Parish" is the last campaign of the five, and the more iconic carnival or mall would've been expected, but hey, they're Valve, I'm not gonna tell them what to do.