When using gdm (init 5) xscreensaver won't work.
It also doesn't work after login as root and using the Gnome Control Center
to change screen saver settings.
I suspect this is a problem with X's security not allowing xscreensaver to
run at the gdm login screen.
Maybe it is a feature and not a bug.
xscreensaver will not work as root.
*** This bug has been marked as a duplicate of 10715 ***