Saying "Microsoft" and "open source" in the same sentence sounds like an oxymoron. It's not. The fact is that while MS isn't exactly sure what to do with open source communities, it would like to attract developers from them, either to proprietary or open source software that runs on Windows. Plus, they have a lot of interoperability issues that need to be worked out. Easing many of those problems are in Microsoft's best interest and they know it, however much they might not want to have to deal with it.
The link above will take you to CodePlex, a Microsoft open source community site. It really is rather cool, if you're prepared to overlook a certain small awkwardness about the whole thing. They're new at it and deserve the break.
You can also view Port25 which is a site by Microsoft's open source labs. Both sites are worth your while.
For Microsoft, it is a matter of adjusting to a new reality of market forces. They are acting in their own interests, to be sure, but that is no reason to disparage their efforts. If they get it right, and there is no reason to think they won't, eventually, we will all benefit.