This would mean that you would have two truly different versions of Windows. Of course, Microsoft has a fetish for shipping 57349587 different versions of each Windows release, but with those, the differences were superficial. If this rumour turns out to be true, however, the ARM version of Windows would be cut off from all the legacy applications that make the Windows platform so attractive to many people.
This new ARM version is, however, the ideal opportunity to cut the cord of backwards compatibility. People probably don't expect to be able to run legacy applications on their ARM tablets, and it would also position these ARM tablets more as competitors to the iPad than as competitors to PCs.
In addition, it would play into Microsoft's supposed goal of converging Windows-proper and Windows Phone. At the moment, Windows Phone is actually Windows CE Compact Edition Embedded Standard Whatever, while Windows-proper is Windows NT. The idea is that Windows Phone 8 (or 9) will become Windows NT-based as well.
While cutting the legacy desktop from the ARM version of Windows 8 would certainly count as a vote of confidence in WinRT and Metro, I'm still not convinced Metro is capable of handling the complex applications and workflows of proper desktops. Especially window management is incredibly restrictive, and limits you to viewing two applications at once - and one of those two will display a more minimal UI. As a translator, I generally juggle a dozen or so windows around, and I know a lot of people who go far beyond that. While this My First Operating System™-approach to user interface design and window management works fine on limited devices like iPads and such, it's hell when you want to get some actual work done.
I've said it before, but I believe the only vote of confidence that will prove Metro can do more than Twitter clients and weather applications is for Microsoft's Metro version of Office to be just as full-featured and capable as the current 'legacy' version. This wouldn't solve the lack of proper window management, but it would at least give me the confidence that Microsoft will eventually get to addressing that as well.
Microsoft expects us to use Metro on our desktops and laptops. They better give us the tools to actually do so, or else Metro is going to be a tough pill to swallow for people like me who actually want to get some damn work done.