I can't recall hearing anything about this, but as I was answering another question this crossed my mind.
With both Windows RT and WP8 running on ARM and having a "shared core," will RT be able to run apps designed for WP8?
You are correct that both run on ARM but the system internals are different.
Besides this the libraries that exist on the phone are different then those for the desktop or RT version.
This being said, if the applications are recompiled and you believe MS, it should be easy to port programs between the two, but this happens not because of the CPUs being similiar but because of the libraires and frameworks being similiar.