from mlb, http://mlb.mlb.com/mlb/official_info/official_rules/replay_review.jsp, part V, G...
Hit By Pitch. Those plays for which there is a possibility that a pitched ball touches a batter, or his clothing. See, e.g., Rules 5.09(a) and 6.08(. Any doubt as to whether a pitched ball hit the bat (as opposed to the batter or his clothing) should be resolved by the Home Plate Umpire at that moment as hitting the bat and not the batter, and the Umpire should declare a fair or foul ball under the circumstances. Notwithstanding the foregoing, whether the ball was in the strike zone when it touched the batter, and whether the batter made any attempt to avoid being touched by the ball, shall not be reviewable. See Rule 6.08(.