I've heard the Harry is a horcrux theory before, and it doesn't make sense to me. Voldemort tried to use Avada Kadavra on Harry, this is canon, Moody confirmed it in Book 4, Rowling confirmed it. Why would Voldemort create a horcrux then try to Avada Kadavra it?
Second the prophecy, which Voldemort clearly puts a lot of stock into as Dumbledore stresses during books 5 and 6, "Neither can live while the other survives". Why would Voldemort put a horcrux into something that he knows will constantly try to kill him and vice versa?
Third, I hear people make the argument that Voldemort did it following the reflection of Avada Kadavra...but we have seen Avada Kadavra in action several times. It kills instantly. There is no chance to do a complicated spell like making a horcrux.
Fourth and finally, it seems Voldemort had been working on creating 7 Horcruxes since his fifth year in school, he entered school in the 1940s, the books take place in the 1990s. I really doubt he waited 50 some odd years to make his 7th horcrux.
The theory just doesn't add up.