It's not as big of a change from the user's perspective, but it is a huge change from the OS's perspective. Replace that antiquated kernel with Win 8 and you have to rewrite everything above it all the way to the silverlight layer. Doing that and writing new device drivers for legacy hardware and testing and debugging all of this so the legacy devices don't experience radio, battery, reliability, and performance issues is *not* a small task. And it's not like Microsoft doesn't have a ton of other things on their plate, like WP8 server, or WP8 on ARM, or WP8 on the desktop, the new Office for ARM, or even the new XBox - lest we forget, Dave Cutler (father of NT) went to the XBox division last year. And if WP8 is supposed to run WinRT apps then you've also got to shoehorn the WinRT software stack in next to the Silverlight stack, all in that same 512k space where you're also shoehorning in the new kernel and all the new features that WP8 is supposedly bringing to the table.