This is good news but little bit too late. Microsoft strategy of sticking with windows to slow down Linux and Mac back fired . If they opened up .Net few years back cross platform app development landscape would have been totally different today.
And who knows, maybe someone will build a decent MSBuild replacement. Or perhaps the Xamarin guys already have a nice alternative.
You might want to check out FAKE, it essentially lets you write MSBuild configurations using F#:
This recent announcement means that a multi-platform FAKE in the near future looks like a good bet.