I would imagine that creating a horcrux isn't an easy thing to do, how did he create one accidentally? And I think that he had a plan for the 7 horcruxes in advance, so leaving one in Harry would be out of the plan. For that matter, everything else that has been considered as a possible horcrux has been an object, a thing. A scar just doesn't seem like a place to put a piece of one's soul. And Voldemort wouldn't have tried to kill Harry in GoF if he knew that there was a piece of his own soul in him.
Sooooo, whatever significance the scar may have, I don't think it's a horcrux.
But if there was a bone-sliver involved, Harry would be carrying Corpse Sickness, and actually fading away and dying without ever noticing, until he's a walking shade. It happened to Scott Glenn in a movie.
(Just a little American Indian tie-in :-).
I'm thinking Voldermort did it by accident when he "killed" his mom and dad and tried to kill Harry. I also think Hermione's cat is either Harry's mom or dad. Probably dad.