As you can read at Bioh4zard2.com, there is hot news from Zac-UK, main developer.
Let me paste it here:
Zac-UK wrote:First of all, L4D2 is released, I’m excited to try this backwards compatability:
Keep in mind, that i’ve only just unlocked L4D2 on steam, and haven’t yet dug around the files to see what we can find out. However I’m slightly annoyed this backwards compatability isnt working out of the box, sure I dont mind spending whatever + hours to get it working, but other users who just want to play their favourite l4d campaigns (suicide blitz or whatever you like best I’ll perhaps update this later today to see if we manage to get anywhere with it. It could be the lack of the SDK i supose, but surely this would require some part of l4d2 to point to the l4d1 directory or whatever.