Earlier quoted context omitted.
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.
They say open source, but not which license they're going to use. It may not be possible to integrate with anything in practice, or as free as developers would like it to be.
AFAIK, it will be MIT licence.
Edit: Yep, MIT. It's not like it's secret: https://github.com/dotnet/corefx