Somewhat. The argument is that "only" the toolkit changes, while the rest of the platform stays more or less unchanged. But of course Qt alone is a very big chunk of the platform a developer gets into contact with. You will still be able to use Gtk/Hildon though, as long as it gets properly maintained and updated by the community. How well such applications will integrate will depend primarily on the extend of the community effort, just as it is the case with Qt on Maemo 5.