I like the sound of it. I have been selling virtues of open source strategy for some time and I know it works.
Facebook will have a convincing claim for leadership in emerging internet operating system if they let other developers run with significant chunk of social networking shell. Shell which hosts all social graph processing. Allow other developers to enhance and implement. This maybe a move to counter Open Social but indirectly it can affect Ning.
Facebook is primarily a database repository. For database company to go open source means what gets distributed will be marginal. This codebase cannot threaten core offering, which is all about enriching and protecting social graph (Scoble is always right!). Which means APIs for read and write can go totally open source but actual data repository will be hard to release. Privacy issues will be real.
If I allow conspiracy theory to run for a while then I see a nice complement to Microsoft's cloud computing initiatives. Also possibilities for tight desktop strategy with WPF/Silverlight APIs linked in. Mother and satellite node picture always gets product managers drooling and I can definitely sense similar temptation here.
With enough user base and third party apps in Facebook.com, they seems to be just ready to pursue bigger architecture ambition.
[Pic thanks to Gapingvoid]