Left 4 Dead 2's sooner-than-expected release, the game doesn't seem to be suffering much. A Valve representative tells Eurogamer that pre-orders for the sequel are currently double that of the original's pre-order figures from the same time last year . Although that sounds impressive, it's possible to see some spin in there -- Left 4 Dead didn't really have its big coming-out party until E3 2008, which technically wouldn't occur for another two weeks from now. Couple that with general sequel buzz and you might have your inflated pre-order figure.
Nitpicks aside, Valve has already stated that it plans to continue supporting Left 4 Dead with more content. The company expects to announce more DLC near the end of summer and has expressed that people will understand the big L4D picture within a year. Assuming it hasn't announced Left 4 Dead 3 by then, of course.