Perhaps Valve really is quietly preparing Linux ports of its games. Rumors of an impending Source engine port popped up over six months ago, and today, Phoronix claims to have located some Linux libraries in the Windows version of the Left 4 Dead demo.
Reportedly, the demo comes with files like steamclient_linux.so, libsteam_api_linux.so, and engine_i486.so—and those didn't make it into the full version of the game. Phoronix went digging around the first file and found strings such as "k_EMsgClientRequestForgottenPasswordEmailResponse," suggesting these libraries are related to something other than the Linux Left 4 Dead dedicated server.
Here's another interesting tidbit: Phoronix says the libsteam_api_linux.so file includes "strings referencing a Steam 3 release client on a Linux file-system (/home/VALVE/alfred/valve/steam3_rel_client/) likely belonging to Alfred Reynolds, who has long been involved with Valve's Linux operations."
Since the Linux files took up "over 50MB" in the Windows demo, Phoronix speculates that Valve could end up distributing cross-platform versions of its games with the right libraries for use on both Linux and Windows.