People need to stop reading what Paul has to stay because Paul is an idiot that does not understand the technical components that he is discussing. His explanation makes no sense whatsoever.
Windows 8 was demonstrated on a single core, 1.2 GHz Quallcom ARM chip
last June 2011 at Computex 2011. That alone suggests the true reason that Microsoft has mandated Quallcom-only within the Windows Phone ecosystem. It makes sure backwards compatibility can be guaranteed in a stable way similar to console development (limits the scope and complexity of development).
Furthermore, that was the full blown Windows 8 OS running. Windows Phone 8 will share the same kernel, but it will not require the same number of services to be running because, well, it's a phone. If Apple can get a pared-down version of the Mac OS X kernel to run smoothly on an underclocked, single core 600 MHz ARM chip, then Microsoft can certainly get their Windows 8 kernel to run on 800 MHz (Tango) processors, and certainly the 1+ GHz processors in non-Tango phones.
It's as if Paul has not even paid attention to many of the goals of Windows 8. Less processes, lower memory requirements, and higher throughput. It's exactly what weaker, battery powered devices need.
I am not sure if Microsoft is going to put Windows Phone 8 onto existing phones or not, but I do know that I will not continue to be a WP7 user if they do not (nor will my Lumia 800-toting girlfriend, nor will my Lumia 900-toting brother). I also know that whatever excuse they might dish out would be nothing but an excuse, just like the excuse people forward for Apple with regards to Siri being iPhone 4S-only.