Valve boss Gabe Newell has confirmed that there will be a demo of Left 4 Dead. Speaking to Videogamer.com, he said,
"We will be releasing demos for both the Xbox and for the PC. I don't know what the date is for release on that, though. I think it's going to contain the first part of one of the campaigns. I think it'll probably be Hospital but I'm not sure. That's a decision that Doug Lombardi [Valve marketing guru] is making."
Of course, Newell doesn't specify that the demo will appear before the game is released. Peculiarly Valve have tended to not release their demos until long after the game is out. The implication here is that there might be a section available before it's in the shops/on Steam, but of course the ambiguity remains.
I was fortunate enough to play L4D in Valve HQ last week, and be assured you should be looking forward to this one hell of a lot. If that's not convincing enough, the opening section of No Mercy should probably do it. Newell goes on to confirm that the demo level will be playable in single player (using bots), and online co-op, which should likely get you a fair amount of free fun.