The company [Microsoft] wants to lure those developers back. Its open source play is aimed squarely at them, and at independent software vendors (ISV). They mean potential sales, and Microsoft has embraced the open source development model in order to tempt them away from the open source platform and over to the Windows platform.
A fairly insightful article on why MS turned up at PDX for OSCON last month. Given the huge market developers could have for any applications they develop using the MS 'Open Source' model this has to something that is taken seriously. The pleasing thing is that MS now sees OSS as such a threat -- however now that the communities have Redmond's attention it is going to be a tough battle for mind share. Let's keep doing what we do and stay frosty.