Assuming this is true, I have to believe that this UI will be an alternative UI, and not a full replacement, or will appear only on low-end tablet-like devices aimed at the iPad. It sounds like something that will silence the critics who want the Windows Phone OS on a tablet. The app store will provide access to new, Silverlight based "immersive" applications that are deployed as AppX packages (.appx). The Windows and Office teams are betting very heavily on this new app type, according to my source, and development has already begun using a beta version of Visual Studio 2012. These apps can be written in C#, Visual Basic, and even C++.
The tidbit about two user interface for Windows 8 is not new, given that we heard similar talk earlier this month. The app store part also isn't new as we heard this as far back as six months ago. Still, new rumors are not enough to prove that old rumors are correct.
Officially, we still only know that when Microsoft CEO Steve Ballmer was asked about the riskiest product bet that the software giant is currently developing, he answered "the next release of Windows" without so much as a second of hesitation. Oh, and also, we now know that ARM support is a go.