This change makes sense. The desktop/metro transition is unwieldy, and ARM is not ready yet for the desktop. It has only just given its 64 bit designs.
Also Windows 8 is not ready for a multiple architecture world. Metro is, using WinRT. But there isn't a universal distribution and application management framework on the desktop that invisibly accommodates x86 and ARM.
Of course what may happen is that, since Metro is suited to tablets, and tablets will be mostly ARM, Microsoft ends up having one OS (Metro) for ARM tablets, one OS (Windows desktop) for Intel (tablets/notebooks/desktops), and one OS (Windows phone 8) for phones. So we will wait for Windows 9 for unification.
Personally I expect to run Windows 8 on an Intel tablet (desktop for desktop, metro for tablet) and also on a phone, but for most consumers I think we'll have the segregation I described.