I picked up a book at Barnes and Nobles last night called "What Will Happen in Harry Potter 7" and read the first few pages of it. They have some interesting theories. The first chapter is about the title of the book. Basically, the Deathly Hallows title is a spin on an old knights tale called the "Four Hallows of the Holy Grail". In that story, King Arthur and the Knights of the Round Table were trying to find the "four hallows" that would lead them to the Holy Grail. These were four items that had magical properties. I'd say its a bit more than coincidence that JKR based this last book at least in part on that story given the name and the fact that Harry has four more horcruxs to attain before finding the "Holy Grail". I'll post more theories as I get into the book.
