Are you suggesting that Voldemort created a horcrux YES 6!, then hid it inside the sorting hat? That is my guess - based on Harry will have to find it and will have to go back to school.--- The Hat has hiden other objects - the sword - and majicians are famous again for pulling other things rabbits out of the hat. If so, then what, the hat never felt the need to share with the headmaster the fact that Tom Riddle had snuck in and put something inside of it? No, He - the sorting hat wouldn't be able to tell Dumbldor - just like the goblit was never able to tell that it had been trick - and if Bones or another student had placed for example the cup into the hat - the hat even if It knew it was having an object placed into it - the hat would not warn Dubmldor as he is not a Hufflepuff. Only a true Gryfindor or Hufflepuff could pull from their own hose an object from their house. It is true that the Goblet of Fire was fooled into thinking that there was a forth school competeing. But actually, we don't know for sure the rules of the tournament, so we can't know how it was confused, or what rules had been tweaked or twisted. The sorting hat talked, the gof didn't. Again we do know that the hat and the goblet were ancient magic as opposed to new magic. Dumbldor said this - and we know that Voldemort set out to find and concure all ancient magic to be the most powerful wizard ever.

Again - We are given clues that Bones will play an important role in book 7, Harry has to find the horcurxes and the hat will play an important part in book 7. Mr. WEasly - be careful of objects that can think for themself if you don't know where it's brain is- let us keep an eye on the hat for Harry-....