Edge Online chatted with Valve's Chet Faliszek about the subject and he states, 'We're still struggling with how interoperability is going to work, right now." Part of the problem is that each game has a different set of characters and settings. Faliszek states, "There's a lot of back-and-forth. We haven't fixed on what we're doing yet, so we don't want to talk about it and make people think something is going to happen when we don't know ourselves." If they don't you can bet mod makers will put in Left 4 Dead 1 maps and characters into Left 4 Dead 2 on their own.
Gallery: Left 4 Dead 2 Screenshots