I was discussing this thread with my DH last night, and he came up with this.
He does not feel the scar is a horocrux because Voldemort would not be trying to kill Harry if it was.
The "Harry is a Horcrux" people explain that away by saying that Harry is an accidental Horcrux.
There are lots of different theories but the basic idea is: Voldemort is planning to make his final Horcrux when he kills Harry at Godric's Hollow. So he brings whatever object he was going to use as the Horcrux with him.
Now comes the tricky part...we don't know exactly how you make a Horcrux. You have to split your soul via murdering someone, but what is the specific process by which you encase the soul fragment? Is it a spell you say before or after you do the killing?
If we assume that you have to prepare the Horcrux before you do the killing, then Voldemort kills Lily, says the Horcrux spell, prepares to kill Harry, his AK spell backfires, so there's a Horcrux spell on the loose and a soul fragment flying around and Harry (or his scar) gets accidentally made into a Horcrux.
Are you with me so far?
Personally, I do not buy this theory. For several reasons, mainly because it just strikes me as too cheesy for JKR.
But using the same logic as the pro-scar people, since Voldemort did not succeed in killing Harry, he would not have torn his soul at that moment, so there wouldn't be a soul fragment to go into the Horcrux. Of course, Voldemort did just kill Lily and James, so we have those soul fragments, but if Voldemort did not designate those fragments for the Horcrux, they wouldn't just go there on their own, would they? After all, Voldemort has murdered dozens, maybe hundreds of people, and only made 6 horcruxes, so clearly you can't make a horcrux just by murdering someone - you have to cast some other kind of spell.
Still with me? Whew. These discussions must sound insane to anyone who hasn't read the books.
I believe, and JKR has said, that Dumbledore is a character that she uses to communicate facts to the audience. Dumbledore said the horcruxes were: the diary, the ring, the locket, the cup, the snake, and something of Ravenclaw's or Gryffindor's. And that's exactly what I think they are.