Neither Microsoft, Nokia, nor anyone else should fork Android. It’s unforkable.
The "android isn't very open source" sentiment really bothers me and I see it a lot here.  It is a complete and fully functional mobile OS under extremely permissive licensing.  Pull it!  Change it! Build it! Fork it!  Whatever!  AOSP is open source.Look at the closed source services google adds.  As far as I know they are all related to google services(Someone please correct me if I'm wrong).  Their store, their maps, their email, their location services.  This isn't needed in the open source distro and it works great without them.  Also there are a lot of restriction on the brand and how you use it when release an Android product.  That isn't unique to Android.  See firefox vs iceweasel.Sure sometimes it gets annoying that a lot of Android design decisions are made behind closed doors and in working in the framework sometimes I have to play the game of "Guess what the Google engineer was thinking" because documentation can be scarce.  Also from what I've heard sending changing upstream isn't an easy process.  These things are nice and make it an easy product to work with... but aren't required for it to be truly open source.  The code is there in a series of open git repos and under Apache license.  That is open source in my book.