When using the strings command to find printable characters within the closed-beta Steam client binary on Mac OS X, there are more references to Linux. Below are a few references.
Assertion Failed: kv
More information (perhaps the best yet) is coming soon. Update: For those thinking these Linux strings are related to just Steam's Linux dedicated server, there are also Linux references to be found within friendsui.dylib, gameoverlayui.dylib, and steamclient.dylib, among other files. Also note this file on Steam's servers.