The theory, such as it is, is that VM transferred part of his soul into Harry at the same time he transferred certain if his powers into Harry (e.g., the ability to speak Parseltongue). That is, at the same time he failed to kill Harry. He'd already killed Harry's parents, so the requisite murdering had already been done. The horcrux could very well be Harry's scar. (Supposedly the last word in Book 7 is "scar." Perhaps the offending sentence will read something like, "for the first time in his life, it was just a scar.")
There is a very short scene in Order of the Phoenix that may support the theory -- where Dumbledore consults one of his odd instruments. Two snakes appear and intertwine, and Dumbledore says something to the effect of "but in essence divided."
I see how that could be argued. But, my thinking is that Tom Riddle was a student when he asked about the Horcruxes and he had asked about the number 7 specifically. I find it hard to believe that he would have not had all 7 Horcruxes made before the night he attempted to murder Harry. I don't think Rowling would throw an 8th at us by some mistake of Voldemort's death curse and inadvertant transfer of powers, etc.
That'll keep me thinking for the rest of the day!