Long we have waited for it. A way to set the Qt graphics system backend without recompiling Qt. In Qt 4.7 this is finally available.
You can configure the backend using the environment variable QT_GRAPHICSSYSTEM.
Now, since the topic of switching graphics backend in Qt is coming up now and then, I thought it would be a good idea to create a nice graphical interface. Actually I wanted something nicer to use for me personally :P
So I created a really simple KCM. You have 3 switches, of which two will write a .sh file to $HOME/.kde/env/. The content of this folder gets loaded at startup (via startkde), and that way you will globally end up with another graphics backend. That said, since the environment variable has lowest priority, it is still possible to override this on a per-application level (e.g. kolourpaint has problems with the raster backend I have been told).