For what it’s worth, Xcode 6 allows you to provide a single PDF file per asset. Xcode will then render the PDF into bitmaps at all required sizes _at build time_ and include the rendered images in your app bundle. Of course, you will probably have to make a new build when new devices come out.

It’s not resolution independence, but it has the potential to make asset production much less painful. I don’t know if this approach also work for app icons or only for things like button and toolbar images.