You can't simply recompile code and expect it to work. You could "port" it, in the more traditional sense, by adapting it to run on the WinRT platform.
Why not? WinRT is just COM under the hood, and COM is just Win32 under the hood.
WinRT is about communicating with the Metro interface and about existing within the restrictions of a Metro process. It's also about being able to use new APIs which understand the permission model of Windows8.
Desktop apps in ARM will be Win32. The only question is whether Microsoft will let you write any.