An alternative to using XNA for .net game/sim development is to go with SDL (simple directmedia layer) (or its .net wrapper assembly SDL.net).
The good news is that SDL takes care of the 2d, input handling, sound, and some networking crap. The bad news is that 3d is outside of aegis, and the best it can do is setup a surface for drawing OpenGL stuff on. That means you need an OpenGL library, which The Tao Framework provides. Bad news there is that it's basically just wrappers for unmanaged code, so you still need to keep track of what you allocated to prevent memory leaks and setup a Dispose() for stuff.
But the sunny side is: works for MS and Mono. \o/
Jabuka has written up some decent tutorials and example code in his physics engine implementation project, which gives some real-world examples of how to use Tao.
So I'm thinking, what, 2.. maybe 3 months for World of Spooncraft 1.0 launch?