Here's my take. I think Microsoft wants to unify their operating systems.
Windows Phone was the first "Metro" experience, but it runs on an old CE kernel and the stack above that is Silverlight (and XNA). Metro is huge. It's the first really new user interface Microsoft's shipped since Windows 95. Metro makes classic Windows and even iPhone and Android feel ancient -- the same old square icons on a desktop we've all been using for the last several decades.
Windows 8 brings Metro to the desktop, laptop, and tablet world. This world, though, is built on the NT kernel, with the WinRT API above that. Sure, you can build Silverlight-like apps in Windows 8 Metro, it might even be trivial to port your WP app to Windows 8 Metro, but you can't easily go the other way.
So, what can Microsoft do about this? Well, it's easy, move Windows Phone onto the NT kernel, and carry over the bulk of the WinRT API. This would make developing your Windows app for any form factor, from desktops to phones, a very easy task. Throw in some nice Visual Studio and Blend templates for re-shaping your app to fit the various form factors, and you've got something really compelling.
The problem with that? Well, today's Windows Phone hardware probably isn't sufficient to drive an NT+WinRT OS. Enter "Superphones."
Superphones, I'm guessing, are the first generation of Windows Phone that run on the NT kernel and support the WinRT (or at least enough of it for most apps.) Note the Apollo release timing is not far from the expected Windows 8 release. Put that together with the recent news that the Windows Phone chief was put in charge of a "a new role working for me on a time-critical opportunity focused on driving maximum impact in 2012 with Windows Phone and Windows 8", and there might be something to this.
So, what do you all think. Am I crazy? Would "same API" across all devices be a worthy Microsoft goal? An achievable one? And what about X-box? Could Microsoft pull off the hat-trick, and unify all of their major platforms under a Metro front end? No doubt that's a tall order, and there are three CPU architectures to deal with. But Microsoft is a big and wealthy company.