Brings up the question of which evil to choose. Do you spread .Net to every platform, but at the same time help further the Linux movement (because if .Net works fine and dandy on both platforms, that means more good apps for Linux as well as Windows.
I know I would develop stuff for it, and might even possibly consider switching completely)
Then again, if you don't push it to other OS's, you are limiting the usage of your (great) platform. Which obviously is not good either.
So, Im not really sure which is better. Personally, I'd like to see the Mono project develop and gain support from MS, but that may not be in MS's best interest. Dunno.