Wouldn't a Shared-Source Windows be Great?

I've been writing a lot about open source and Microsoft lately. Ryan Rogers, a Microsoft employee, wrote a post back in July on that subject. It has drawn a lot of attention, but I wasn't blogging back in July, so I didn't see it until now. He suggests something beyond anything I had imagined - a free open source Windows OS. Go read his post here.
After reading his article and many of the comments, I wondered if a shared source version of the Windows core OS might be more realistic.
I have benefitted greatly from the .NET Framework shared source -- and I know the whole community has benefitted too. I personally would be highly interested in a shared source version of the core Windows OS.
Tweaking one of Ryan's ideas, it would also be cool if , "Microsoft decided to release the next version of Windows (after Vista) as a project like Fedora" under a shared source license for the purpose of letting researchers contribute to improvement of that next OS while also enabling those researchers to learn about and understand Microsoft's work.