In development, using V 1,1 of the framework, we have seen cases where Combobox.SelectionChangedCommitted does not fire. Ever trying numerous work arounds, I finally asked MS. Here is their reply:
" It sounds like using Alt DownArrow to pull down and up the droplist -- which currently (and in 1.1) does not fire selectionchangecommitted. And yes, there are many places where the SelectionChangeCommitted event isn't fired even though the value has changed. SelectionChangeCommitted only fires when the user hits enter or clicks on the item. It should have the exact behavior as in everett. We were going to change this event, but not only would it be a breaking change, but the meaning of selectionchangecommitted would be blurred with selectedindexchanged. If you think of how IE works, you can Alt-Down to select another web site, and then Alt-Down again and IE does NOT navigate to the site. However, if you hit enter or click on the item, IE will navigate."