I'm not sure the portable class library actually applies in this case, but even so you still need an application to call that library.  All of the info I have seen leads me to believe that you can't create a desktop app with WinRT and that you never will.  There are a bunch of things in WinRT that only apply to a Metro environment.  At some point, we may see a desktop equivalent of WinRT that contains the relevant parts without the Metro stuff.  That would be nice.  It seems we are going to have to wait for Win9 to see what the plan is going to be for the future of the desktop.

One of these days Felix will probably uncover what is going on ...