In an interesting but highly speculative article at TechCrunch, the idea is put forth that we'll be able to run Windows Phone apps on Windows 8--why you would want to do that, we're not sure. The thrust of their argument revolves around "Jupiter", Microsoft's upcoming developer framework for Windows 8:
"Jupiter may end up being the “one framework” to rule them all. That means it might be possible to port the thousands of Windows Phone apps already written with Silverlight to Windows 8 simply by reusing existing code and making small tweaks. Or maybe even no tweaks. (That part is still unclear). If so, this would be a technical advantage for developers building for Windows Phone 8 (code-named “Apollo” by the way, the son of “Jupiter”) or Windows 8."
Of interest is to note the Roman mythology here: Jupiter is the father of Apollo, with the latter being the code-name for Windows Phone 8. We suppose this would be highly useful for developers for somewhat obvious reasons, though the benefit to consumers would not seem as strong. Windows 8 is expected to feature its own app store with programs designed for that user experience and even if WP7 apps could run on Windows 8, scalability issues will abound. Still, it's not hard to think that yes, in some form this could be accurate as we're expecting for Windows 9/Windows Phone 9 to merge into a single system--but that's still a ways off.