Ok, heres my theory. In the maze. . . Hary encounters an acromatula. Which according to "Fantastic Beats and Where to Find Them" utilizes a deadly, poisonus, secretion. If you recall Harry noticed a "secretion" next to the OPEN WOUND on his leg. . . Theres been sufficient evidence that Harry in immune to poisons (he was bitten by a basilisk after all his body with the help of phoenix tears has probably developed the immunity.) Well. . . we must take into account how much DD knows about the maze, and what was in it. It's probably safe to assume that he knew Harry had to encounter an acromantula.
Anyway, Voldy used Harry's POISONED blood to create his new body!
One of the biggest arguments against this theory, is that the accromantula was not put into the movie. . . but do the writers know whats gonna happen in later books?