There are some annoying issues and crashes in KDE 4 (and particularly in KRunner and Nepomuk) that are related to the thread-safety (or, rather, thread-non-safety) of libdbus-1. kde-core-devel has seen some discussion about this recently.
There is a patch that fixes the issue, but it won’t make it into D-Bus before dbus-1.4, due to the fact that it changes the library behaviour and could potentially break bindings. Thiago has confirmed that the change doesn’t break the Qt bindings, and (from the bug report), no-one has found any other bindings it breaks.
This is all just background on why, if you are running Arch Linux, you might want to check out the dbus-core-mt package on the AUR. It’s dbus-core-1.2.24, as found in core, but with the afformentioned patch applied. Note, however, that you use it at your own risk – it should improve KDE, but it may well break other stuff.