I encountered some Qt blocker problems on a system I rarely update (once per three months maybe). I asked Ben de Groot (yngwin) about that, and there's a nice one-liner that worked for me and may be useful for you:
emerge -1av $(eix --only-names -I qt- -C x11-libs)
It seems that all of Qt packages must be at the same version, and for some reason portage only wanted to update some of them when I ran emerge -uDNa world.
Of course before running that command make sure that eix database is up to date with eix-update.