Xamarin is awesome
I LOVE Xamarin, and think this could be a good move.
UNLESS... MS decides to kill off Xamarin proper in favor of absorbing it in to its other existing products. Its not hard to imagine MS absorbing parts of the Xamarin GUI in to Visual Studio, and extending the actual .NET framework with some of the mobile specific classes that are in use behind the scenes. I fear that the end result would lose all of what makes Xamarin so cool right now.
Another aspect that could get weird is the whole Mono thing, which Xamarin uses. Would they replace the underlying Mono framework with the real .NET classes and extend Xamarin to use all the .NET classes instead of the subset that Mono replicates?
More I think about it, the more this makes me, as a Xamarin fan, nervous.