Cant see it happening, given the amount of resources M$oft has put into this, I cannot see them giving it away by making it cross platform. As far as M$oft's mission is concerned having .NET is suicide. Remember that they are competing against Linux with Windows XP, so this kinda conflicts with their interest as in soo many cases.
My conclusion is that .NET will remain Windows specific - a way to stop current long suffering VC++ and VB developer from moving to Java, theyreby limiting Windows' Homogenity.
As for C# - Anders, if you're going to copy something, at least copy it well. It seems Mr Heilsberg is still using his Borland experience - Object sender (nonesense event listener model compared to Java) - not suprising given that he cant learn anything new at MSoft. Just look at how far Borland has come since this Anders H left - Kylix, JBuilder. Anders is still stuck to his Delphi / T Pascal days.
So as a cross platform developer C# can pass on ....