Has this been explicitly stated? I'm hoping they do unify under the single WinRT framework.
EDIT: just said that the CLR is now identical between phone and W8, but again "share more code", still not naming WinRT.
If that's what they're doing, I think it's the right decision - the UI conventions themselves are different between the phone and Windows 8 (more than people realize, despite having a similar visual aesthetic), so you'd have to rewrite much of the UI anyway - so why break compatibility with WP7.x.