On the game page there is no Mac tag[1]. Also, someone mentioned in the forums that, "POrtal 2 would be their first game to be simultaneously released for Mac and Windows: http://store.steampowered.com/news/3569/[2].
I imagine that Portal 2 and Alien Swarm are based on the same version of the Source Engine that hasn't been ported yet. All the Mac releases so far are older games on older engine versions. Both Left 4 Dead games haven't been ported to Mac yet.
