Anyone who buys the game on Steam will get all of the DLC for free, which makes for the original five campaigns plus the three DLC levels to play through. That's an excellent deal for sure.
Unfortunately, the first game is not yet ready for the Mac; apparently, it's taking Valve a little longer than expected to get it OS X-compatible. But Valve promises that when it is released, Steam owners will get the same deal -- all the DLC with the game itself. That's good stuff. Of course, with Left 4 Dead finally coming to our platform, which game will we be stuck waiting for next? Civ V, where you at?
Thanks, Ben H!