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...)

FWIW, I've had success running Steam under Wine. Have only tried CounterStrike:Source but it works reasonably well.

Football Manager and CS Source work as long as you don't run it with an extended desktop over 2 screens.

Valve, if you are reading this, ROCK ON!!!