Big update:

1. new wiki hierarchy, see http://wiki.maemo.org/Qt5-Maemo5
2. debs are available in the repo (see wiki), no need to unpack tars any more
3. all builds are performed from within scratchbox, so you can use Qt5 in the same way as Qt4 from scratchbox
4. QtMaemo5 module and QMaemo5Style (now part of the module) ported to Qt5 (but Maemo5 style is not the default style!), so widget based Maemo5 Qt4 apps may be ported with fewer efforts
5. In the aforementioned qt-project thread someone mentioned a similar problem to the QtQuick Text item bug while debugging on Windows. The poster promised to provide some example soon.
6. My repo (see wiki) also contains some test apps. You may suggest more apps, but they either have to be Qt5 apps or you must send me the ported source code. Currently there's only a widget based desktop build of trojita (package name trojita-tp). Try it with different -style and -stylesheet options.
7. I'll not have much time during the next 2-4 month, so development will slow down.

Some details for the further interested:

Don't use QMaemo5Style directly as this now requires the private headers (e.g. QT += widgets-private in your .pro file). The package qt5-maemo5 contains a style plugin in the next version, so use QStyleFactory::create("maemo5") instead of QMaemo5Style();

Porting the required QGtkStyle (see repo) back to gtk 2.14 (vanilla Qt5 claims it would need 2.18) was a matter of replacing function calls with macro calls. The way Qt loads the library functions from Gtk2 is quite interesting: Qt requests pointers to functions and call those functions later, without checking for null pointers. So an old Gtk2 version causes no compile time errors and the application silently segfaults. Since most of the required gtk calls are present, simple example apps even worked before I backported to older gtk, because the null pointers were simply not called at all.