From the Microsoft announcement:
Through Xamarin Test Cloud, all types of mobile developers—C#, Objective-C, Java and hybrid app builders —can also test and improve the quality of apps using thousands of cloud-hosted phones and devices. Xamarin was recently named one of the top startups that help run the Internet.
We have had a longstanding partnership with Xamarin, and have jointly built Xamarin integration into Visual Studio, Microsoft Azure, Office 365 and the Enterprise Mobility Suite to provide developers with an end-to-end workflow for native, secure apps across platforms. We have also worked closely together to offer the training, tools, services and workflows developers need to succeed.
With today’s acquisition announcement we will be taking this work much further to make our world class developer tools and services even better with deeper integration and seamless mobile app dev experiences. The combination of Xamarin, Visual Studio, Visual Studio Team Services, and Azure provides a complete mobile app dev solution that provides everything you need to develop, test, deliver and instrument mobile apps for every device. We are really excited to see what developers build with it.
All of which has to be good news for the Windows 10 ecosystem as a whole, with more and more services and projects appearing for the platform.