With Qt 5.1.1 testing there were bugs like Ubuntu's Weather and Calculator apps on Ubuntu Touch turning black, the Unity 8 Shell crashing, etc. Rather than going after fixing any problems in the tool-kit, the adoption of Qt 5.1 was postponed until after the Ubuntu 13.10 release.
Ubuntu 13.10 shipped with Qt 5.0 instead of Qt 5.1, since while it was available for months prior there were some "unfixed regressions" in the newer tool-kit version. For Ubuntu 14.04, Qt 5.2 is being targeted that will be available in release candidate form in the next few days.
After the official Qt 5.2.0 release (planned for December but given the delays might not come until January), the Ubuntu QA team is looking to do a Qt 5.2 PPA and run some Ubuntu-focused testing. There might also be some work in preserving compatibility for existing Qt applications with the Ubuntu Touch Click packages.
In terms of supporting Qt5 on Mir, the notes from today's virtual Ubuntu Developer Summit covering the Ubuntu Qt support mentions, "qtubuntu with Mir support a separate QPA module on which Qt will not directly depend on. It will be aimed for desktop at 14.10, while it's currently in use in Ubuntu Touch 1.0 already." So Canonical is still not planning to (try to) mainline their Mir support in the Qt tool-kit but will be shipping the display back-end support as a separate QPA module.
While Ubuntu Touch is already using Qt on Mir, Ubuntu 14.04 won't be powered by Mir, so as such they're not looking at the desktop side support for Qt Mir until Ubuntu 14.10.
More details on the Qt5 Ubuntu planning can be found via this vUDS notes page.