The Gnome-Appearance-Properties (GAP) manager for Hardy Heron (System>Appearance) has a bug in it, where if there is specific information in it's configuration file then it will hang after closing, causing huge CPU consumption. You can check this with the System Monitor very easily.

The problem is caused by a theme line in ~/.gtkrc-2.0. Comment this theme line out and all of a sudden GAP closes happily.

Example:

#include "/usr/share/themes/Glossy/gtk-2.0/gtkrc"

Bug report here: https://bugs.launchpad.net/gnome-control-center/+bug/147045

The bug has already gone upstream, as it belongs to Gnome.