Dental fluorosis is a condition resulting from a child ingesting too much fluoride while the permanent teeth are developing. In its mildest form, faint white specks can be seen on the teeth. In more severe forms, the teeth can appear brown and mottled.
Dental fluorosis has been found to be more common in children that consume fluoridated water. One source for this claim is this article published in the Journal of the American Dental Association in October of 2010. http://jada.ada.org/content/141/10/1190.abstract