How to fix Ubuntu Unity in VirtualBox

Currently I'm playing around with Ubuntu 11.04 Natty Narwhal in VirtualBox. However I stumbled accross the following error: Shortly after startup and sometimes during usage Unity falls back to a old fashioned classic theme. To restore the default look, run the following command in a terminal:

killall -9 gnome-settings-daemon && gnome-settings-daemon && nautilus -q

The following screenshots illustrate the situation before...

... and after running this command:

Thanks to webupd8.org for this solution.