Detect hardware buttons on Android, how?

They are dispatched as regular key events. The following script handles the standard menu and back buttons:

 function OnGUI() {
     var e = Event.current;
     switch (e.type) {
         //case EventType.KeyDown:
         case EventType.KeyUp:
             switch (e.keyCode) {
                 case KeyCode.Menu:
                     OnMenuButton();
                     e.Use();
                     break;
                 case KeyCode.Escape:
                     OnBackButton();
                     e.Use();
                     break;    
             }            
             break;
     }
 }