When faced with the choice of killing the "one who could vanquish" him, and saving a piece of his soul, V chose to kill Harry. (He'd already made horcruxes and was obsessed with immortality.)
So he AV's Harry. Lily's sacrificial mother love causes the curse to bounce off Harry and render V helpless and bodyless.
However, Harry still has the soul piece in him. Maybe Lily's love forced it out of Harry through what is now the scar on his forehead??
There are also the 24 hours between the events at Godrick's Hollow and when Harry is brought to the Dursley's.
In HBP, the horcrux ring worn by DD has a crack down the middle that was caused when DD destroyed the soul piece residing within it. I wonder if there is a similarity between Harry's scar and the crack in the horcrux ring??
That's a good point. I'm still not sold on the whole Harry as horcrux theory, though. I'd buy Snape's a super double-double secret agent before I'd believe Harry a horcrux. Although, anything is possible. But like others have said, why try and kill part of your own soul (as in Voldie repeatedly trying to kill Harry) then?