These files included the Steam Linux client binary and all of the other Linux libraries that are needed for the client. These files haven't just been sitting around, but Valve has actively been updating the Linux-specific bits as we noted yesterday. However, exposing Steam / Source is indeed coming over to Linux, prior to Valve announcing it, has generated a fair amount of attention around the Internet.
Unfortunately, as of today, Valve has pulled the index file from their server and the related bins_linux.zip and steam_linux.zip (or they have just been updated and due to the lack of the index file we don't know the new checksums). Hitting the steam_client_linux file now results in a 404 error. This is the file used to monitor the different Steam Linux components and to know whether any of them have been updated.
Looks like we'll have to wait for an official Valve announcement before we can do any other file exploration of the Steam Linux client.