Looks like there is a real possibility of an official Steam Linux client being in development by Valve with some keen eyed Phoronix reader finding the file steamclient_linux.so packaged with the now expired Left 4 Dead demo.  Note this has nothing to do with Steam Linux dedicated server binaries.

We were tipped off this morning by an attentive Phoronix reader that in the demo copy of Left 4 Dead there were several Linux library files worth investigating. In particular, one of these files was even named steamclient_linux.so. This is in the Windows demo client -- we're not referring to the Linux dedicated server-only version.

Steam has possibly held together the non MMORPG PC gaming sector with its constantly elvolving Steam platform and making Valves baby more Operating System agnostic seems like the next logical step forward.  Source