Green tea gets a good review in the Journal of Periodontology. The March issue includes a study which analyzed a number of lifestyle factors for 940 middle-age Japanese men, including tooth brushing, smoking and green tea consumption. It found that participants who drank green tea had less periodontitis or gum disease. Periodontitis is a chronic inflammatory disease that affects the gums and bone supporting the teeth. It's caused by bacteria in the mouth and is made worse by poor oral hygiene.
Source: tatoeba (12211125)