I stumbled across this the other day, I've no idea why the fedora team would go to the effort of making a really nice theme for the lock dialogue for gnome screen saver then leave it "disabled by default". After a quick google I found the ubuntu guys have been up tosomething similar, so here a quick how to enable the fedora art work on the gnome-screensaver lock screen.
To enable paste this into a terminal (NOT root, as your normal user).
gconftool-2 -s --type=string /apps/gnome-screensaver/lock_dialog_theme system
To switch it back:
gconftool-2 -s --type=string /apps/gnome-screensaver/lock_dialog_theme default
The only "downer" is that your avatar isn't shown, but for my work laptop I prefer this theme, much cooler fedora advocate when I'm not at my desk ;)