Don't forget that C# guys are usually Microsoft bred. Windows guys (in general) just can't seem to grok any other OS or command line tools while non-Windows guys can do ok in Windows (but they complain a lot about not being as productive as they could be). So hiring Java guys to do C# is probably easier than the other way around, but they might want more for the pain of having to deal with Windows.
Edit: There are of course exceptions to this, but I have seen it too many times to not see believe it is the norm.
Windows guys (in general) just can't seem to grok any other OS or command line tools
I have to take exception to that -- or at least the specific phrasing.
It's certainly true that most MS-platform guys don't grok other platforms. But I strongly disagree that they cannot do so, given a brief period to learn.
Almost everyone I know who does Windows dev work used to do Unix work. All you do in college is Unix. And most university recruiting is for Unix positions. Those .NET enterprise jobs don't recruit at colleges, but no college student knows C# or .NET or ASP.NET.
My experience is these people typically transition over later in their careers.
Edit: There are of course exceptions to this, but I have seen it too many times to not see believe it is the norm.