Steam confirmed for Linux.
http://www.phoronix.com/scan.php?page=article&item=valve_steam_announcement&num=1
http://www.telegraph.co.uk/technology/apple/7715209/Steam-for-Mac-goes-live.html (last sentence)
http://www.phoronix.com/scan.php?page=article&item=valve_steam_announcement&num=1
http://www.telegraph.co.uk/technology/apple/7715209/Steam-for-Mac-goes-live.html (last sentence)