Reasons harry might be the 6th Horcrux:
1. Harry is clearly connected to Voldemort in a supernatural way that seems to need a better explanation than "Voldemort tried to kill him."
harry has the feather of the same phoenix as Voldemort in his wand. And how else would Harry be able to speak snake. his parent's couldn't, but Voldemort could. And of course, Harry could sense Voldmort's feelings in book 5, and as said already, see through the snake in book 2.
2. It would be a perfect reason for Voldemort to almost die that night. He had finally broken his soul into the magic number seven parts. This would probably kill any wizard. (also Dumbledore thinks Voldemort had not yet made the sixth horcrux that night)
3. Voldemort had just committed a murder (Lily Potter) and so it was the right moment to create a horcrux.
4. This would explain why Snape so often protects Harry. He does not let Malfoy hurt Harry at the end of book 6 and says he belongs to Voldemort. A coinciding theory would be that Snape had a special mission from Voldemort to keep Harry Potter safe at Hogwarts.
-I don't think that Harry will have to kill himself to kill Voldemort. I bet Harry will be able to change the part of Voldemort that is within him with love or something along those lines. Harry's soul, pure and strong, will be able to defeat Voldemort's shattered soul inside him.