Hyper, Super ... I'm talking about mod4 (on most keyboards the win-key right to L-CTRL). The overlay is normally activated by pressing mod4. With the stated option ticked, triggering the overlay with mod4 doesn't work anymore (when I remember correctly) - but every other shortcut using mod4 + anykey works again.
To get to the overlay using a shortcut, I set it to mod4 + a.
Ah, right. That's probably a nicer way of doing it since you don't have to mess with gconf-editor.
Personally, I cleared /apps/mutter/general/overlay_key (which prevents the overlay from being triggered by any modifier, which got my mod4 bindings working again. I set /apps/metacity/global_keybindings/panel_main_menu to "<Mod4>Escape", which on GNOME 2.x activates the Applications menu, but on GNOME 3.x activates the overlay.
To get to the overlay using a shortcut, I set it to mod4 + a.