Android is a slim Linux distribution, and iOS is a scaled down OS X, yet Windows Phone 7 has a totally different set of internals than the desktop Windows line. There have been rumors swirling for some time that the Windows desktop line and Windows Phone OS will be merging. If so, Windows 8, with its focus on touch, mobile, a WP7-like development experience and runtime, and ARM CPUs seems like the opportunity to do this.
I think Windows Phone 8 is quite likely to have Windows 8 under the hood, but I could see it waiting until a later date. What do you think?