I always heard that when that happens, it is an automatic triple although I can see the umpire giving the batter a homerun if he feels that the ball would have been over the fence.
http://mlb.mlb.com/NASApp/mlb/mlb/official_info/official_rules/runner_7.jsp rule 7.05 Each runner including the batter runner may, without liability to be put out, advance_ (h) One base, if a ball, pitched to the batter, or thrown by the pitcher from his position on the pitcher's plate to a base to catch a runner, goes into a stand or a bench, or over or through a field fence or backstop. The ball is dead; APPROVED RULING: When a wild pitch or passed ball goes through or by the catcher, or deflects off the catcher, and goes directly into the dugout, stands, above the break, or any area where the ball is dead, the awarding of bases shall be one base. One base shall also be awarded if the pitcher while in contact with the rubber, throws to a base, and the throw goes directly into the stands or into any area where the ball is dead. If, however, the pitched or thrown ball goes through or by the catcher or through the fielder, and remains on the playing field, and is subsequently kicked or deflected into the dugout, stands or other area where the ball is dead, the awarding of bases shall be two bases from position of runners at the time of the pitch or throw.
