[rpeden]

A quick look at the ASP.NET MVC source shows that it uses the Apache license, which is OSI approved:

http://opensource.org/licenses/category

So yes, I'd say that is sounds like open source, using a very permissive license. People can (and do) deploy ASP.NET MVC apps using OSes other that Windows. The vast majority choose to deploy on Windows servers, but that does not make the source any less open.

I don't mean to come across as a Microsoft apologist; I've worked with .NET, Ruby, and Java...and find them all acceptable ways to get things done quickly, once you know what you're doing. I just feel like if we all spent less time navel gazing about whose language/tech stack is the most awesome, and spent that time creating things instead, we'd all be a lot better off.