Orbit and Dui are UI Frameworks which provide ways to write finger oriented, highly animated applications. Basically something similar to what iphone, android or palm pre provides. QWidget is not designed for that. They also provide quite a few other things like virtual keyboard, localization, application lifecycle, service framework, notification framework and so on. Perhaps I phrased it wrong in my message. Dui apps are not maemo specific, they are Dui specific. You can take Dui library to your platform (where qt is supported) compile and create nice looking applications. Developers would want to use them because of current trend in application development. People don't buy crappy looking apps. They want eye candy. Eye candy is something which is not easy to achieve with QWidget. Looks like QML is coming to rescue but with QML DirectUI can and will work quite nicely, so they will be nice additions to each other and not really competing.
Btw, DUI is going to be heavily optimized for OpenGLES 2 devices. Not sure i can say the same about QWidget apps.

Dui/Orbit QGraphicsView based applications. You can mix QT into those framework if you can inject QWidget into QGraphicsView. Actually you can do that with proxy widget but it will be pretty slow. Check recent Gunnar's post on QT blogs.

Noone tried to include Orbit and DUI widgets in same app. I don't see reasons why they can't coexists. They both based on QGraphicsWidget. How much #ifdef will be required depends how you structure your app, so hard to say. Lets see what is Orbit in details, what layouts and widget it has, what api it uses and so on.