I think the difference is between people who consider themselves "Microsoft developers" and those who consider themselves ".NET developers". I never quite understood the former; it's sort of like Java developers calling themselves "Sun developers". Microsoft developers don't bother to learn anything unless it's branded with the Microsoft logo, whereas .NET developers recognize that the .NET framework and the associated languages are simply tools in the toolbox. Sometimes it's the right tool for the job, sometimes not.
There is a small but growing minority of .NET developers that are interested in open source. In fact, I've now incorporated questions about open source into my questioning when we interview new developers. Just because someone has an MCPD doesn't mean they can code their way out of a paper bag. A bunch of commits to an open source project does, though.