The Milwaukee Bonsai Foundation has a collection of some of the finest trees created by Milwaukee, Chicago and Midwest artists. The collection is maintained at a pavilion on the grounds of the Lynden Sculpture Garden. If bonsai are viewed as being "sculptural art" by the Lynden Sculpture Park, that is good enough for me to accept bonsai at its higher levels is Art.
So since sculpture artists consider bonsai art, I figure I can consider bonsai an ART.