For the first time in all the time I have been using Arch I have experience my first major package breakage.
The night before last I ran pacman -Syu and one the packages upgraded was the Qt package. It was upgraded to qt-4.8.1-2. Next time I booted up I was getting a KDE Crash dialog appear noting an error with KRunner. Trying to run krunner from the shell I was presented with a host of errors.
A quick search of the Arch forums I found this post reporting similar problems. Following the advice in the post I downgraded the Qt package and my problems where resolved.
To find out if you can downgrade your package you will need to search the /var/cache/pacman/pkg directory to ensure you have a package to downgrade to. If you do then you can run pacman -U /var/cache/pacman/pkg/qt-4.8.0-6-x86_64.pkg.tar.xz (or whatever version you have). If you can not find an earlier package in your cache then you will have to refer here (the Arch wiki) which will provide further details.
(I should mention that I am running 64bit although I have no idea if this effects 32bit).