Sorry, this post will be rather long...
Ok... Open your copy of "Harry Potter and the Sorcerer(sp) Stone" (Or Philosopher Stone) in he last page of chapter VII. (Funny: in the portuguese version, it's on page 111!)
Now, read Harry's dream. We know dreams have a further meaning, at least in HP world, cause he dreams about Sirius flying motorbike, and it's during the sleep they are most able to receive teh "energies from beyond"... (ok.. I didn't find out a better expression!)
He was wearing Quirrel's "hat" which was saying him to go to Slytherin. At first, it might seem that it's only related to the Sorting hat, when it said Harry would be well in Slytherin. But I don't think it's just that: Quirrel's "hat" (sry, I don't know the real word) is a symbol for Voldemort. Does Voldemort want Harry in the Slytherin team? It seems really unlikely: Voldemort wouldn't care in which team Harry is. But... What if Snape is bad (and only if he is bad), and Voldemort knows it? Would he prefer Harry in Slytherin, so Sanpe could spy on Harry?