Aaron Siego wrote a blog post this morning about Qt 5 and KDE 5. The official KDE short answer is that it's not yet known whether Qt 5 will be greeted by the release of KDE 5. As mentioned yesterday, there shouldn't be many compatibility breaks between Qt 4 and Qt 5 so the KDE 4.x applications should be able to build and run fine or with just minor revisions.
The topic of KDE 5 with Qt 5 is also being hotly debated right now on the KDE development list.
The last time KDE 5 was talked about in public when the idea was expressed of merging KDE's libraries withg Qt.