Left 4 Dead is the last promised Valve title to see release on Steam for Mac, and yet we haven't heard anything about an official release date. Valve was moving along pretty well for a while there, releasing games for our platform every Wednesday. But they've petered out lately, leaving a lot of Mac gamers wondering if we'd ever get to shoot zombies along with the rest of the gaming world. But a slip found by our former cohort David Chartier, now at Macworld, hints that we'll be exploring the zombie apocalypse soon -- he notes that Valve is bringing the game's new downloadable content, "The Sacrifice," to all platforms, including the Mac, by October 5th.
The Sacrifice is an extra level that's available for both the first and second games in the series, so there's still a chance that Valve will only give us the original Left 4 Dead, and save the sequel for later. But I'd guess the reason for the long delay is that they're bringing both games over, so we might see both of them released on Steam for Mac by then.
Of course, this could just be a slip of the blog, with the Mac just accidentally listed as an available platform. But that doesn't seem likely -- Valve tends to be careful about what they say. To whet your appetite, check out the game's brand new comic, created just for the new DLC.