I think this is a great move for both Xamarin and Microsoft. The company I work for is currently using Xamarin to help create cross platform apps with a consistent backend architecture. Sure, we have to develop different front ends for each platform, but we can leverage C# for everything and we don't super stars on any one platform to allow us to generate things quickly and well.
I was waiting for something like this to happen, ever since I watched this video from Hanselman at the Xamarin Evolve conference last year: http://www.hanselman.com/blog/XamarinEvolve2013TalkVideoHowC...