Sadly, Microsoft is one of the reasons C# isn't living up to its full potential. If they would release an official version of .NET for Mac, Linux or at least officially support Mono project in a more decisive way, C# will become a lot more popular. I don't think they gain anything by this though (apart from Moonlight which lets them advertise platform compatibility), which is why they will never do this :(
Even though MS is a large money making behemoth, I don't necessarily think C# would be served any better by MS trying to support other platforms. A large part of .NET depends on a well supported class library. These class library can be very operating system sensitive. Backslashes vs forward slashes, directory separators, nonblocking IO, GUI stack etc. It is hard to do this well. Particularly on a platform that belongs to your competitors.
Microsoft has pushed Silverlight as a .NET vm that runs on Mac, with a smaller framework library. So far, this has met will little traction. I don't think it will encourage MS to push this further and wider.
