How do they port it to Linux, then? (They have to re-compile it right?)
I've developed games before and I can tell you, if you're not using propitiatory platform-dependent libraries, porting your game is a piece of cake. What I've noticed is that most of the games on Humble Bundle use a library known as SDL (Simple DirectMedia Layer.) SDL is a library that handles two main things that aren't platform-independent: creating a window and receiving events (keyboard, mouse, e.t.c.) The rest is taken care of by the cross-platform OpenGL, OpenAL, etc. libraries.
In addition, there's an even better library than SDL called SFML (Simple and Fast Multimedia Library). SFML is much more fully featured than SDL, and is C++-based and OOP. With both SFML and SDL, all you have is _just recompile_. And done!
So really, it's not that hard to port a game. "can't justify sinking man-months of effort into a Linux port" sounds a bit stretched. There are plenty of free libraries available for game dev (atleast 2D game dev), like Box2D, CEGUI (Crazy Eddie's GUI), etc. so if from the get-go you plan on supporting multiple platforms use some of those libraries, and porting will be a breeze.
