Internet Explorer on Windows has IMO a nice solution; when the toolbar 
doesn't fit, it adds a button at the end of the toolbar labelled 
">>" that drops down a menu with the remaining items. Assuming you use 
GtkMenu for that, it's pretty easy to implement. This does break things 
slightly, since toolbar buttons get reparented. Also, the toolbar API may 
not be conducive to making this work, I'm not sure.

Random thought: this problem also needs solving for menubars. Playing with 
IE, I don't see any way in which menubars and toolbars behave differently; 
the "items" are just slightly different in size. So maybe we should have a 
single menu/toolbar widget. Some of the buttons even have drop-down menus, 
including the ">>" button and the "Mail" button; i.e. the buttons are just 
like menu items. This is not counting the buttons that have a button on 
the left and a drop-down arrow on the right (e.g. "Back"), the mail button 
is actually only a menu item, clicking it anywhere drops down the menu and 
has no other effect.

But that's not a 2.0 thing, unless it's the easiest way to fix 
the "toolbar doesn't fit" bug, which seems doubtful.