or... you get the sdk and you re-implement it for the mac.
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].
[1] http://store.steampowered.com/app/630/
[2] http://forums.steampowered.com/forums/showpost.php?p=1611274...