"Valve has gradually developed the once-despised platform into
something worth supporting, but if there is any one remaining flaw
of the system, it's that the service is not cross-platform
compatible. However, if some snoopers who have found Linux binaries
in Valve code are correct, that may need a qualifying "yet."
"Some crafty penguin-heads have come across a bit of interesting
code buried deep within the Windows client for the Left 4 Dead
demo. According to phoronix, the Windows demo contains a number of
shared libraries for Linux, including a number of APIs that are at
least named to suggest they could play a key role in a
Linux-capable version of the game and Steam itself..."