With Qt it will be very simple for Nokia to backport new core applications to Maemo5. The main problem is multitouch-oriented interface, but I hope they will keep on using resistive (and so non multitouch) screens on some devices. In this case they will make singletouch version anyway.
I know power of Qt: on my site you can see my application running on 4 different systems. It's made from single code, but interface must be tuned for different sizes of screens.