Microsoft is now a major sponsor of the Apache Software Foundation and is putting its protocols and formats into a royalty-free license, all part of a larger open-source push.
Big things are happening with Microsoft lately. As an Open Source developer by nature I can say that I was a little shocked when they first opened their Microsoft Office document format specifications (which I was sad enough to read!). There are various news stories where Microsoft is becoming more open. I like it. I like it a lot.
As to why they are doing it, the only reason I can see is that they are taking a beating in the the computer industry with competitors like Apple, IBM, Mozilla and Google all peaking at their profits. Not to mention the European computer laws on monopolistic behaviour. They need to form alliances and change market opinion about them if they want to remain unchallenged.