It’s definitely happening but it’s still being planned. Gabe says:

“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 [game's four] campaigns. That’s a decision that [Valve marketing director] Doug Lombardi is making.”

Well, if you were at PAX then you know how long the line was to play Left 4 Dead. It was more or less the same as what I played at E3 so I didn’t want to delay someone else’s good time by getting in line, but people were having a blast. I’d say it was one of the longest lines in the place, up there with with Gears 2, Fallout 3, and Starcraft 2.