When ribbon items exeeds teh width of the window they are displayed on, a button should appear at the far right of the ribbon with a little arrow on it (button drawn over ay existing item.)  When the user will place the mouse cursor on top of that button, the ribbon items should scroll, just as if the user would scroll teh mouse wheel.

A similar behavior, albeit reverse, should exist on the left, when the ribbon gets scrolled and items are pushed outside the left of the window.