If a gamer with plenty of time in his hands (think teenager and/or young adult) is a bit techy-hungry or just techy-curious, that person will download and try to install a Linux distribution.

If on his freshly installed free OS he can *very easily* install Steam and start playing any of the Steam game in no time with proper voice support and Steam-community support, that guy will think: "Hey, this is cool!". Nothing else is cooler on Linux for someone that age. The stability, the OS performance, the virus-free hassle, the constant kernel optimisation etc, is just an added bonus but nothing as cool as going online and play Left 4 Dead (or any other Source-based game) on your freshly-installed totally-free new Linux OS. It is then, and only then that the desktop market will start to really change.

Dual-boot is a pain in the neck. Virtualisation isn't strong enough to run a AAA game within Linux at decent FPS and Wine is only a temporary solution (sometimes working very well, sometimes not so much).

If Valve is thinking about bringing a native Linux version of their gaming platform, they're simply investing on making sure they can still sell their games no matter what tomorrow's OS platform will be. Funnily enough, I believe that if they release a native Linux port of their gaming platform, they will incidentally be a key factor in bringing more people to Linux, by thousands if not millions.