There are actually some strong OS voices in Microsoft. I was at Solutions Linux in France (around 2006 iirc) and there was a Microsoft booth. Since our booth had some downtime I talked to the guy manning their booth who was kind of stranded between Linux distros and FLOSS companies.
He was pretty cool and genuinely trying to advance OS within Microsoft but said it's a pretty frustrating experience overall (his descriptions of the internal processes at Microsoft were pretty interesting). Seems like they have come a long way since, I hope he's still working there. I should have his card somewhere at home :)
So as far as I know there have been developers who were pushing to open source a lot of infrastructure/language stuff for quite some time.
Edit: I think it's no coincidence we see this happening now that Ballmer is gone. He was kind of the villain in the "let's open source stuff" stories I heard.