Valve today has confirmed that they will be a demo for Left 4 Dead and another surprise to this zombie twist is that the demo will also contain co-op for some zombie blasting fun.

"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 is making."

Newell confirmed that you'll be able to play single-player and co-op in the demo.

Source