Open-sourcing .NET doesn't magically make it work on other platforms (a not-insignificant chunk is Win32-specific, for example). But it certainly makes the Mono team's job easier.
> and expanding .NET to run on the Linux and Mac OS platforms.
http://news.microsoft.com/2014/11/12/microsoft-takes-net-ope...
AFAIK, it will be MIT licence.
Edit: Yep, MIT. It's not like it's secret: https://github.com/dotnet/corefx
Source: http://blogs.msdn.com/b/dotnet/archive/2014/11/12/announcing...