Are you kidding?
QT is like a game engine, u can't say, oh lets play Minecraft(what ever ^^) in cryengine 323, it has to be rebuild/rewritten for new all the new interfaces. (Correct me if I am wrong and tell me when apps, compiled for qt4.x libraries, magical work with qt5.x)
when ivo777 says something, it usually means a lot, to me at least, however I'm not all that willing to fresh flash my N9 at this stage, like even just to see....
How do you explain the appearance of beta then? If it did not make sense developers of QT would you do it?
It's been explained like 10 times already in this thread:
Programs compiled against Qt 4 cannot and will not use Qt 5 libraries.
They are not binary compatible. Actually they are not even completely source compatible.
Yes, Qt 5 is an improvement over Qt 4. Yes, Qt 5 can be installed along Qt 4.
But no, there isn't a single program on a freshly flashed N9 that uses Qt5: you would need to change the sources of the UI and main apps, and then recompile them for you phone to use Qt5.