Actually, the VS project file format is standard. Mono supports both Visual Studio formats (.sln, .csproj, *.fsproj, etc) as well as Makefiles
I'd actually be very interested in this. Recently went back to C# and have quickly remembered how painful .sln and .csproj files are to merge >_< Would love a solution that used something easier to merge, lol!
With that said, most .NET developers probably prefer their .NET OSS projects to have visual studio files so its easier to link into their projects.