Reading all the previous comments, I think they bring up interesting points. First, I ask this: do we know that Voldemort used Adava Kedavra on Harry? Also, do we know that the Adava Kedavra spell is used to create a horcrux? I'm not sure if it was ever specified, just that the act of killing mangles one's soul to enable the capture of it in an outside object. I think the brevity of Slughorn's conversation with TR was completely intentional so we would hit this junction as to whether or not a horcrux could be created unintentionally. My gut reaction is yes, it could be, because I think to kill with the intention to then make a horcrux requires a specific spell. There have been other wizard murderers, why haven't they made horcruxes as well if it jut required the desire to remain immortal?

I think James is the descendant of Gryffindor, which is why Voldemort, the descendant of Slytherin, decided to kill his child who, like him, had one muggle parent. I think something with Lily's sacrifice created a magical contract- these, we've come across in GOF but werent too well explained- which, by Voldemort acting to break it, the action failed and the last of his soul departed his body, part of it entering Harry and the other wandering off to Transylvania or wherever. I think Harry's scar is where Voldemort's soul shard entered his body, and when Voldemort is near or emitting emotion Harry feels it in the scar, as the soul wishes to rejoin with him as the concept is unnatural.

Sorry that was ridiculously long. Oh, and one last thing- since JK had divorced, I think it would be like her to implement that in the wizarding world, marriages or other magical contracts were inalienable on penalty of death. Have you seen any instances of divorce in Harry's world?