http://www.linuxjournal.com/content/resetting-gnomes-settings-ubuntu
Basically, if you've been customizing and tweaking and somehow end up completely borking things this can undo all changes (like a nucelar option) and set it back to it's default state. This is just for GNOME settings, it won't help you repair damage you did to graphics drivers, x-server, etc. Just GNOME.

