Well, there is Mono. Mono is an open-source project that works on developing just-in-time compilers for the .net framework for different platforms. Using Mono, it is possible to port C# applications over to other platforms. (such as Mac OSX and Linux) It's a proven project with commercial examples. The Unity game engine and editing system was created using Mono, and they've ported it to Windows, Mac OSX, Linux, and even have it running on the Wii.
The biggest problem is, obviously, DirectX. Even if the C# code can be ported to other platforms, DirectX can't be. It's Windows only, and there's only one real other option. If SGDK2 is ever going to be cross-platform and have hardware GPU support, it's going to have to integrate OpenGL. No two ways about it. If SGDK2 gets OpenGL support, it will be viable for cross-platform porting. As it stands, you will have to be content with the 90% of the market userbase that Windows provides. Boo hoo.