Wow looks like one of the whiners from the Steam forums made it to this board too. 'Give me back my money because my demo is delayed by a few hours'? That's laughably ridiculous.

For what it's worth though, I think they held it back for some technical reason. Maybe they know it will spike bandwidth usage and they are releasing it at the time when their average usage is low? Also, there was a Steam client update just now and when it restarted, the L4D2 demo preload started downloading some data too. It's possible they put out a patch for the game even though it's still unlocked, which could mean they found some bug or something in the demo. Or it might simply be a matter of them always having planned to release the demo at this time and someone made a simple mistake in the Steam community announcement.