Valve has said that Left 4 Dead owners on Windows and Mac will be able to join forces or go head to head cross-platform in multiplayer.
It's all thanks to the new Mac version of Steam, due out next month, which will be supported by the Mac-ification of the entire Valve catalogue, including both zombie shooters.
Thanks to Valve's "Steam Play" feature, Windows players who also have Macs don't have to worry about forking out for a separate version either. As the Left 4 Dead blog points out, "if you own Left 4 Dead 2 for Windows, then you already own it for the Mac."
"We'll be releasing the exact system requirements soon. The Mac versions will be updated in sync with the Windows versions, so the platform wars can continue uninterrupted."