Cough sources! Hamachi worked fairly well last night, though Luc found it somewhat laggy. Ideally on Friday Night we might be able to get 4 players on. That way if you add in the fact that there is no more computer AI to help you out, and the randomness of getting 4 human players on it should be utter mayhem.
Also any CSS, TF2 etc map will work, seemingly the game will automatically spawn in zombies and kits etc.