Hi radiohead: Hagrid is a good guess although JKR mentioned that the clue to the Half Blood Prince is in the Chamber of Secrets book.

In the movie, Aragog is careful to mention that he came to Hogwarts in the pocket of a traveler. In the book, Aragog says that the traveler was from a distant land and he gave Hagrid a spider egg to care for. But who knows? Maybe the prince is Hagrid after all and the spider is just pulling our legs.