I run my X server with the AllowDeactivateGrabs and AllowClosedownGrabs options
set. This allows misbehaving clients who grab the server to have their grab
deactivated and to be killed, respectively.

If I hit Ctrl+Alt+Keypad-Divide when the lock dialog is on screen, the lock
dialog loses focus permenantly and I can't enter a password to unlock the screen.

If I hit Ctl+Alt+Keypad-Multiply, the screensaver process is killed and the
desktop is revealed.

xorg.conf(5x) states:
"Note that the options AllowDeactivateGrabs and AllowClosedownGrabs will allow
users to remove the grab used by screen saver/locker programs. An API was
written to such cases. If you enable this option, make sure your screen
saver/locker is updated."

So it appears that gnome-screensaver needs to be updated to use this API. Of
course the man pages don't seem to mention this API anywhere, but xscreensaver
cannot be deactivated or killed with these keys, so the answer is at least in
xscreensaver's source.