making menuitems insensitive by app context

[Rob Benton <rob benton conwaycorp net>]

I'm looking for a way, maybe a signal, to make my menuitems 
sensitive/insensitive based on the state of my program.  Using 
gnome-terminal as an example, something like:

Edit->Copy

only being sensitive when you have text highlighted.

Is there some way I can capture the click/button_press of the Gtk::Menu?