I'm not so sure the OP was talking about web development. As has been pointed out we now have mono and if one is interested in web development it will work with Apache. I have also experimented with using the MS .NET stuff with Apache on windows but that is a discussion for elesewhere

Right now I would say C# is going to get you more work for a while as it is still relatively new with fewer programmers than c++ and Java, but with quite a lot of PHB brainshare - I see quite a few roles where they are also looking for experience with other MS software such as BizTalk and SQL Sew^hrver - but with some good rates (at least in the UK right now).

/J\