The open-sourcing is a sign that MSFT is in a tricky spot (BYOD, mobile, tablet, games) and desperately needs to improve relations with the outside world. Expect we'll also see MSFT using more open source in products to compete effectively though I wonder what'd happen with internal best practices that the outside world doesn't have, e.g. SAL. Will they contribute back? Let's see. Inside MSFT there used to be minimal…
It's also a sign that Microsoft sees its future in services. Which in that case means that protecting the former "company jewels" of Windows and Office ceases to be less important.
Microsoft realized they were becoming to FOSS as OS X is to Windows and is trying to counteract that. Microsoft's vision is still valid; they want Microsoft technology running every computer in the world.