Valve rolled out their "Steam 2010 Client" to Windows gamers today, which brings a new user-interface and other improvements (here's the details for those interested), and is also what their forthcoming Mac OS X client is based upon. Hidden within this major Steam client update are more references to Linux.
As we're actively testing Microsoft Windows 7 x64 on a number of systems at the moment for next week's Windows vs. Ubuntu vs. OS X benchmarks, we installed Steam's new Windows client on one of the systems. After the Steam client was installed, Counter-Strike: Source and Half-Life 2 were installed before switching back into Ubuntu Linux to run the strings command against these files to see what would turn up.
While Valve has been working on the Linux client and we know its coming, over the weekend they pulled the Steam Linux client from its servers, but there Linux references remain within the Mac OS X beta client and today's 2010 Steam Windows client. This is all after we originally found mentions of Linux in their Mac OS X version.
Even the Steam.exe Windows binary features a string that reads "Bad eCurrentLinuxClientVersion field in CClientConfigRecord" with additional references in the steamclient.dll and SteamUI.dll files.
Deeper though there are also Linux references within friendsUI.dll, gameoverlayui.dll, ServerBrowser.dll, SteamService.dll, SteamService.exe, and vgui2.dll. These are client files, not for the server support.
In regards to the Source Engine's support for Linux itself, within the base source engine 2.gcf that is a common game cache file used by Source-powered games on Steam, there is a "os" "string" // WIN32, LINUX string. Within steamapps/source engine.gcf is the same string again plus "Linux Specific Data:" and another reference.
Let's hope Valve officially acknowledges this work soon and announces when they plan to release this native Linux client.