Why would Valve cut the demo for a game they're trying to market?
Maybe there is a more than a hint of truth about demos stealing sales. Or maybe they'll just leave the single player mode open to showcase the title without giving away the real substance of the game, which is the multi-player team mode. Although they're still giving that away with Zombie Panic!, just without the plot and characters.
Not sure I agree with that entirely since I pre-ordered the game through Steam after playing it, even though the demo didn't exactly scream "best FPS of the 2008 holiday season." Just very solid technically, and very fun to play, which is really all that a game needs to hit the mark.
Either way, the demo's been working since the 6th. The only demo I can't get to play on Steam is Tomb Raider Underworld for some reason.