If the option to unredirect fullscreen windows (/apps/compiz/general/screen0/options/unredirect_fullscreen_windows) is enabled then the gnome-screensaver lock dialog is not visible and you are allowed to view windows underneath by pressing alt-tab.
unredirect_fullscreen_windows cannot possibly work on the Fedora version of compiz since we use the a feature of Composite called an "Overlay Window". This window would cover any unredirected fullscreen windows and cause all sorts of unpredictable behavior. So WONTFIX'ing this bug.