We have a release date for the Left 4 Dead demo. It's available to all from 11th November, running until the 18th when the full game is released. However, if you want more than a week's free play of a couple of levels from the game, Valve are offering more as a pre-order bribe. Anyone who pre-orders via Steam (or in participating US stores) will get access from 6th Nov, giving you five extra days of four-player zombie destruction.
We don't know for sure, but a sensible guess would suggest the demo will show levels from No Mercy, the series of levels set in and around a zombie-infested hospital. That's the area of the game Valve have been most keen to show off so far.
I'm surprised they're switching the demo off immediately on launch. Even if they put two separate levels from the chapter in the demo, with as much replay value as that would offer, I'd think people would want to be able to play to the rooftop finale and then other three chapters once they'd got a taste for it. I don't imagine this would be a Quake III, where people happily play the demo alone for the next decade. Valve do as Valve does, and it seems to be working for them. But you'd think the real buzz for the game would appear post-release (even if they are spending a mind-hurting $10 million advertising it), which would then attract the larger numbers of new people wanting to see what it's all about.
Unless there's more surprises in store. Who knows.