GameIndustry.biz did an interview with Doug Lombardi, the Marketing VP of Valve Corporation. In that interview Lombardi has put an end to all the rumors by saying that they are not working on Steam for Linux right now.
Excerpt from the interview:
Question: Final question, and one I’m sure you’re not super-keen to answer, but I promised one of our tech guys I’d ask it. What truth is there to rumours that you’re also working on a Linux version of Steam?
Doug Lombardi: There’s no Linux version that we’re working on right now.