Gnome Shell makes use of a single application menu that is always present in the top bar. It gets its contents via GtkApplication. In the current stable version nothing really uses it, but for GNOME 3.2 that will probably be changing. For example, a mockup for GNOME Contacts has an application menu that is mandatory to use the application to its fullest: http://afaikblog.wordpress.com/2011/06/09/presenting-gnome-contacts/
Are there any plans for how Unity will accommodate this?