I can't give a reliable opinion on the usefulness of Qt since I'm not an engineer, but as far as my layman understanding goes, it's a UI toolkit and as such pretty irrelevant for games that tend to have their own UIs - you would just use Qt to set up a canvas for drawing with OpenGL ES. Zen Bound doesn't use Qt.

If Qt provides a layer of abstraction for realtime user input (multi-touch events, tilt sensors etc.), and abstracts the system audio and provides a mixer for streaming buffers, abstracts interrupts events and elegantly handles system graphics overlays then it probably would reduce headaches for cross-platform game development. If it doesn't, then porting games to Symbian will be as painful as always. As said, personally I haven't researched it and my opinion is worth a peanut

We currently have no plans regarding Symbian devices. That may change, of course.