Also, he marked Harry as his "equal" (according to the prophecy), when that cannot be the case if Harry is a Horcrux, since it would mean that Harry possesses only 1/7 of Voldemort's soul. However, Dumbledore only had 5/6 in mind (the snake, and the three relics from Slytherin, Ravenclaw, and Hufflepuff), so it's entirely possible that another person is a Horcrux. Snape, perhaps, seeing as he's the most controversial character in the series.