I have just realised in Ubuntu Brainstorm (http://brainstorm.ubuntu.com/idea/120/) that a program called BTNX can help users who have a mouse with additional buttons get the most of their mice.
The interface to it, however, is quite complicated and not as simple as we would like for a Gnome app. It is quite undiscoverable, too, just because a user trying to configure their mouse is naturally going to look in system → preferences → mouse.
I think having a simplified frontend to BTNX in the mouse preferences menu would be the perfect solution. I have made a quick mockup:
As you can see, the interface is heavily influenced by the keyboard shortcut menu. The idea is, you hit "detect a button", then click the button you want to configure; then you can give it a name, an effect -key combination or command- specify the input for it.
