ETA 02.13.12: De acordo com JK Rowling, o Chapéu Seletor de fato detectou a porção da alma de Voldemort dentro de Harry:
"I always imagine that the Sorting Hat detected the presence of that piece of soul when Harry first tried it on, because it's strongly tempted to put him in Slytherin. So that's how I see it." JK Rowling - Pottercast 12.23.07 - The Leaky Cauldron
Além disso, JKR diz se Harry é ou não um Horcrux adequado:
"Here is the thing: for convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make," but I think, by definition, a Horcrux has to be made intentionally. So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux, but Harry did not become an evil object. He didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul."
[SNIP]
"I do think that the strict definition of "Horcrux," once I write the [Harry Potter Encyclopedia], will have to be given, and that the definition will be that a receptacle is prepared by Dark Magic to become the receptacle of a fragmented piece of soul, and that that piece of soul was deliberately detached from the master soul to act as a future safeguard, or anchor, to life, and a safeguard against death." JK Rowling - Pottercast 12.23.07 - The Leaky Cauldron
Então, sim, o chapéu detectou a porção da alma de Voldemort dentro de Harry. Mas não, Harry não era uma Horcrux oficial. Eu nunca fiz isso antes, mas vou escolher minha própria resposta porque é diretamente do J.K. Rowling, e eu levei tempo para realmente pesquisar a questão além da minha opinião pessoal / interpretação do texto e do HP Wiki.
Concluo que o Chapéu Seletor conseguiu detectar a porção da alma de Voldemort em Harry. Harry não era uma Horcrux oficial. Não é estabelecido em cânone se o Chapéu Seletor pode manter conversas convincentes sobre uma pessoa com outra.
E eu sinceramente peço desculpas a JackBNimble : /