Harry is a Horcrux

Ok,

Therory time. Harry Potter and The Order of The Phoenix is where I first rememebr noticing the clue - The prophecy states that only harry or voldemort may live. The worlds not big enougth for the two of us sort of thing.

Clues in other books,
Philosophers Stone - Voldermort Kills Harrys Parents but not Harry.
The Half Blood Prince - Dumbledore explains to Harry that for Voldermort to split part of himself into a Horcrux he must kill.

Theres many other references and I'm sure if you think about it you will see them thru out all the series, you know the whole - "I'm so much like Voldermort" teenage whining thing Harry has going on.....

ETC