Here is a link to the source of the information but if the wall of text intimidates you then you will be please to know that The Verge have summarised the story for your viewing pleasure.
It sounds like an intentional information leak from Valve, that is if its not entirely fictional. If Steam really does officially launch for Linux then its a dream come true for many Linux driven gamers as well as none gaming supporters of the operating system as it if a firm step in the right direction if Linux is ever to become perceived as 'mainstream'
I have ran many Source based game's on linux and had little issue making them run through the Wine software however a native version would be very appealing to many gamers however I am not inclined to believe this story on face value at the moment.
It's also worth remembering that this is not the first time that a linux based steam has been talked about but until I see a post signed with 'Gabe' I'm not putting any stock in it.
UPDATE: This is gaining legitimacy by the second, even RPS are confirming this