I wouldn't doubt that Valve was working on a Linux Steam client and perhaps a port of the Source engine to give it some content. But at this point I think it's kind of jumping the shark. In that these could be tentative and experimental development. Valve to my knowledge has yet to announce their intentions. Considering how much of an investment Steam and Source would be on Linux especially when you consider that it would have to be supported into an unknown future time I would take all of this with a grain of salt until Valve puts down some official intentions.
No announcement yet.
At This Rate, Don't Be Surprised If You See Steam Soon