From the Left 4 Dead “The Sacrifice” update page:
Left 4 Dead 2 will be released on the Mac Tuesday October 5th. If you already own it on the PC, then you own it on the Mac. All you need to do is install it on your Mac and start playing.
If you don’t already own it, now is a great time to pick it up. Not only do you get Left 4 Dead 2 but steam owners get all the previously released DLC for free! So you get the original 5 Campaigns and “The Passing”, “The Sacrifice” and “No Mercy.” Best of all, there is already a thriving community and excellent community created campaigns waiting for you.
Sorry Left 4 Dead owners, the Mac version is coming but we are running a little late. We hope to have it out in time to celebrate Halloween on the Mac. When released, L4D1 Mac players will receive the same great treatment. All the previous released updates and DLC for free and the ability to play an extensive library of community content. More information on the release date as it nears.
When Valve introduced Steam for the Mac, they promised to bring all of their Source engine games over to the Mac and it looks like they are continuing to deliver on that promise. They also introduced SteamPlay, which allows those who have purchased a compatible game on the PC side to download and play it on the Mac side free of charge. So if you’ve already got a gaming PC and already purchased Left4Dead 2, you don’t have to buy it again just to play it on your Mac. Simply login to the Steam service on the Mac using your account, and the game will appear in your Library, ready to install.
If you haven’t yet signed up for Steam, do check it out and support Mac gaming.