Looks like Valve is definitely going towards multi-platform support. While the Linux version doesn't do anything as of yet, one commenter got the OS X version running after somebody pointed out that both
http://store.steampowered.com/public/client/steam_client_osx and
http://store.steampowered.com/public/client/steam_client_lin... are valid URLs. Both clients download updates, and the OS X version pops up the new, unreleased Steam application and everything. However, it appears to be in closed beta, since he was kicked off after trying to log in (see
http://www.reddit.com/r/linux/comments/butl8/steam_for_linux...)