TheJoe: This is one of the reasons why we have a problem; people still think Linux is an operating system. It isn't, it is a kernel; a component of the (by convention) GNU system.
If you want to be nit picky about it.
However, I don't know many people that get just the kernel without an additional GUI or a shell :P and those tend to repeat themselves from one distro to the next.
And anyways, desktop games usually interface with the kernel only and some graphics library like openGL, making the distinction doubly moot.
From there, you can make a distro specific installer, but that's less than 1% of the work anyways.
TheJoe: Developers don't need to think about creating "a game for Linux", they just need to be platform agnostic and the best way to do that is
to release your goddamn source code and let us take care of it.
Developers need to learn that there is no profit to be made from their code. There is no threat to their game's integrity. There are absolutely 0 negatives. If you're a good developer you'll create a good engine that is clearly separate from and useless without your assets (see id Tech 4). You don't even have to support a source release; John Carmack has given 0 fucks about his code releases, we, the public, took care of it, built our own documentation and went to town in our own way.
Making games cross-OS compatible is not witchcraft (at least, not between Windows and Linux).
Basically, the more the platform sticks to the C++ standard and put the platform specifics into Kernel calls, the easier it is. Windows and Linux's main compilers (Visual Studio and g++) have done a good job with that. Console-specific compilers (usually modified gcc)... not as much.
You just need to use the C++ standard library as much as possible (which got greatly expanded with C++11) and for the rest, you use wrapper classes (or typedefs) inside of which you can put precompiler branches to compile under various platforms.
Either way, I'm a big fan of open source for general purpose things (OS, browser, etc) where having something that is open and free becomes doubly important, but I'm not sold on it being viable for very specialized software (like games)... at least not on release.