You don't have to learn about POSIX and OpenGL to program a game on Linux. For example, could use KidsRuby (which was recently featured on the Raspberry Pi blog) + Gosu. Take a look:
http://confreaks.net/videos/637-gogaruco2011-kidsruby-think-of-the-children?player=html5
PyGame is also very popular for getting started with making games:
http://pygame.org/news.html
There are tons of libraries like Gosu and PyGame that make game programming easy. Plus, the advantage of using libraries like that is that by the end of it, not only have you got a game, you've also taught yourself a versatile programming language for more practical tasks.
All valid points, but nothing beats banging the hardware directly which Gameduino seems perfect for.