I have good news and I have bad news. The good news: A Left 4 Dead 2 demo is on the way! The bad news: Unless you're going to buy the game anyway, you can't have it. (At least, not right away.)
Valve is unleashing the demo for Left 4 Dead 2 on October 27 on both Steam and Xbox Live, but there's a catch: If you want it, you have to preorder the game. According to Game.co.uk, customers who don't preorder aren't completely out of luck; Xbox Live Gold members will gain access to the demo on November 3, while Silver members will get it on November 10. Still waiting on confirmation of that statement from Valve, however, and as yet there's been no word on how PC users will be handled.
I guess I'm getting old, because I remember a time when a demo was a tool to entice customers to buy a game, not a reward for doing so. People downloaded demos to see if they actually liked this game or that and wanted to throw some cash at whoever made it. Of course, that's not really a viable option when the formula is reversed and you must first throw cash, then get demo; and while I can't help but think that they've somehow got it backwards, I suppose that criticizing Valve's business acumen at this point is a bit of a foolish proposition.
UPDATE: Valve has confirmed that there will be a public demo released for Left 4 Dead 2 but specific dates have not yet been announced. Not what you'd call a huge surprise, perhaps, but still nice to hear it from the source.