When you configure Compiz keybindings through the gnome control center, and you pick a shortcut that applications are already using, the compiz shortcut should override the application shortcut. This does not appear to happen currently. For example even if you set maximize vertically to F11, gnome-terminal will still go to fullscreen when you press F11.
but the keybinding capplet does not allow conflicting keybindings, or does it ?
The compiz keybindings aren't in the gnome-keybinding-properties capplet (AFAIK), and the keybindings for the terminal don't appear in there anyway.
well, "maximize vertically" is a window manager keybinding, and it is certainly there.
Should be fixed in -30.