Pressing the power management button in the screensaver preferences dialog does nothing! Notice that I'm not running gnome-power-manager, my install/homedir predates gnome-power-manager and gnome-power-manager has never been added to my session. IMHO the capplet should detect if gnome-power-manager is running and if it isn't not show the button.
I'd just make the button insensitive, but yeah.
I've filed this upstream: http://bugzilla.gnome.org/show_bug.cgi?id=424165
Fixed in 2.18.0-6.fc7