Developers over at Phoronix have found evidence in Steam's Mac OS X launcher that Linux support is in the works and it will be coming not to far from now.
"The launcher checks the platform so that the appropriate library path can be added to the respective environmental variable for loading Steam's shared libraries needed by the client and then launches the Steam library, which prompts another conditional platform check."
The above process is indicative of Linux portability, or why else would they put it in there?