Since the Mono project really didn't get a huge following, I think making .NET open source really isn't a big risk for MS but executes a calculated gesture of good will. I don't expect a OS X .NET compiler of any use any time soon. We will still be using MS .NET rather than Bob's .NET. Open Source done right (for example the JavaScript libraries, like JQuery) is a very good thing for the developer community. Linux got into so many variations that it just got silly; like a graveyard of rusted out cars.

But it will generate a [good] stirring in the fringes, and generate a better understanding of how the framework works for those who really want to know.