I believe there are enough Linux gamers out there to justify a Linux port. Valve obviously thinks the same. The rumored Steambox will run some variant of Linux. Plus, if you already have a Mac OS X version of your game, in many cases your work on a Linux port is halfway done. When Unity finalizes its Linux version, the door will be open for many more games.
Personally, the only reason I use Windows at all is because of the games that aren't playable under Linux. As soon as a significant portion of my library is playable on Linux either natively or via Wine, I'll be kissing Redmond goodbye.