Can we expect to see a high quality C# compiler for non-Windows systems in the foreseeable future? I know that both Java and C# have been submitted to ISO for standardization or something....Sun maintains compilers and virtual machines for various platforms. While there is a GNU Java compiler, my understanding is that it is not able to compile all Java code. In other words, is does not support all the same "libraries" that Sun's compiler supports.

I don't expect MS to develop compilers for *nix and Macs. Will the open source community be able to develop a compiler that supports all of the languages features?

On linux (and on macs as well) mono supports c# and even has quite a few windows libraries ported

http://www.mono-project.com/Main_Page

Be aware that Sun's Java is being released under the GPL (in stages I believe). Not sure if that process is yet complete.

monodevelop supports c#.
eclipse supports java.

Is Mono being used for commercial purposes on Linux and Mac systems? Is it being used to compile 'mission critical' programs? I would expect that Sun's Java implementation would be preferred over Mono in this circumstance.

http://www.mono-project.com/FAQ:_General
http://www.mono-project.com/Companies_Using_Mono

Yes, Java would most likely be preferred, since it is far more widely used. SUN is open sourcing as much of Java (including the client and compiler) as possible and have been for some time.

http://openjdk.java.net/