Microsoft clearly is involving Xamarin in this announcement -- parts of Xamarin are going into the same dot net Foundation that's in charge of this, and de Icaza was on stage for part of this. It may be best for Microsoft at this point if they support and involve Xamarin without buying it outright, people who want to use Xamarin with MS tooling can take advantage of Visual Studio integration, people who are wary of Microsoft can use Xamarin Studio and Mono and never officially touch Microsoft's stack while still becoming part of their ecosystem.

They announced a Foundation with members outside of Microsoft, in charge of a mix of their code and code from Xamarin. So you need to coordinate all of that. You also probably need to have lawyers review all the code and make sure that MS has the rights to everything included so they can do this, someone to scrub all the comments to make sure developers didn't throw in something embarrassing or worse, you need to set up workflows for reviewing community bug reports and patches... this isn't something you can do in an afternoon.