There has been no confirmation that W8 and WP8 will be the same OS, and in fact the rumors from quasi-reputable sources have been to the contrary: W8 and WP8 development are currently proceeding separately and WP8 is still CE-based; they want to eventually merge the kernels but that is planned for further out, Windows 9. (though I'd imagine whatever plans they have for that far ahead are tentative and probably open to heavy revision once they get around to actually working on
(a more realistic and interesting possibility though is that the new Windows Runtime/Jupiter framework in W8 will be ported to WP8/CE, which could make it easy to write (new) apps that work on both. There is some evidence of this in the leaked W8 builds as they include support for phone dialing.)
Yeah, I might have accelerated the timeline a bit.
Even so, I think it's clear that they're going in the direction of merging code-bases, and I'm certain that touch-screen Windows 8 and Windows Phone 8 apps will be cross-platform and be based around the same high-level framework. It'd be insanity otherwise.
(a more realistic and interesting possibility though is that the new Windows Runtime/Jupiter framework in W8 will be ported to WP8/CE, which could make it easy to write (new) apps that work on both. There is some evidence of this in the leaked W8 builds as they include support for phone dialing.)