Well, Deep Silver and 4A Silver can never be called exclusionary. They've officially released Metro: Last Light for Linux on Steam, plus the Mac version to Apple's own App Store.
In addition to that, the 300 lucky individuals who are helping Valve test the prototype Steam Machines will find a copy of Metro: Last Light included. 4A Games has promised full support for Steam OS and the Steam Controller, which was unveiled and detailed in September.
Oles Shishkovstov, Chief Technical Officer at 4A Games, commented:
"We are excited by the prospect of bringing the ultimate Metro experience, powered by next-gen PC hardware into the living room. Metro: Last Light will be a great showcase for Steam OS and the Steam Controller."
Currently, Last Light does already support Steam Play, meaning if you own any version of the game on Steam, you'll find it added to your PC, Mac, and Linux Steam libraries automatically.
In early October, Valve released the specs for their prototype "Steam Box", and many of us drooled a little on our keyboards. Mind you, the hardware may not ever see the light of day as an actual console in its current form, but we're curious to see what comes out of all this testing.