Mono isn't just a c# compiler, it is also a CLS Machine (virtual machine) and a set of libraries, among other things. If the VM was also put under apache 2, it is also unlikely that it would run on Linux out of the box.

To use java as an example, this would be like open sourcing javac without open sourcing the JVM. (Just fyi, both javac and JVM are open source under the GPL license. :) [edit - Looks like large parts of .NET platform are being released]

Still, this is a move to be applauded. Microsoft has been easing itself into a more open source friendly company over time by putting more projects under an open license. Microsoft ASP.NET MVC is another good example. There is an incomplete list of open source projects originating from Microsoft here: http://www.microsoft.com/opensource/directory.aspx

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Miguel just demo'ed building a C# iOS app using a modified Roslyn-powered C# compiler with Xamarin Studio on OSX.

Miguel showed personally on stage they integrated it in mono (in a version that, obviously, at the moment is not out there)

This is still Windows only. If anything Mono now has more resources to pull from to improve their project.