@AndyC: I think he's talking about applications like Internet Explorer, who have both a desktop UI and an immersive one. IE gives you the opportunity to switch to the desktop one from within the Metro one. I don't think our metro apps are able to do that.
Even if we could do that there is still the problem about having different update cycles and install cycles for each version of the app. One of which will not be able to be updated through the windows store.