I know this is an old thread, but I thought it deserved a kick.
I'm just recently starting to really play with my Raspberry Pi, and thinking of all the educational value it would have for my boys (ages 8 and 12). I'm thinking of helping them each build their own setup.
I'm also thinking that C# is a dramatically more sensible language for them to learn than Python. In fact the older one is already pretty deeply into C# on the command line (doing Project Euler challenges) and in Unity (making 3D games). (The younger one is still doing Scratch.)
So it'd be super-cool if they could write games on the RPi itself, in C#. One approach I'm considering is to simply wrap the OpenGLES APIs directly, along with OpenAL, in some 2D framework that's similar as possible to Scratch (and perhaps bears some resemblance to Unity where that makes sense).
But another approach that looks really promising is MonoGame. I haven't tried it myself, but I see from this thread and the videos that people have gotten it to work, and to perform well... but it appears to still require a lot of fairly deep hacking.
So, long story short, please let me encourage you all to continue working on MonoGame on the RPi. I think it's a tremendously valuable contribution, and I'd love to see all the rough edges ironed out, so that anyone can do a couple of apt-gets and start programming games in C# right away!