[Warning: If you haven't finished all of HP through book 6, but you plan to, do not read this post, because it contains plot details.] To follow up on Ilya's post to kick off the weeklong build-up to Harry Potter 7...I recommend that serious Potterphiles check out HogwartsProfessor.com. Some very sophisticated analysis. On this page, you'll see links to buy some books--which I urge you to purchase with expedited shipping, so you can read them this week, and thereby understanding Book 7 in greater depth when you start reading it at midnight on Friday. "Who Killed Albus Dumbledore?" and "Unlocking Harry Potter" provide diverse analyses of the mystery, of Rowling's literary techniques, and of the omnipresent influence of alchemy.
At the least, these books demonstrate quite persuasively that what Harry (and the naive reader) saw on the Astronomy Tower in the climactic scene of book 6 was certainly not the full explanation for what was really taking place.
My own analysis, "Severus Snape: The Unlikely Hero of Harry Potter book 7" was originally published on the VC in 2005, and was cited by the NY Times a few weeks ago. Russian, Polish, French, and Spanish translations are available.
A few further predictions:
1. Especially given the alchemical necessity of a resolution involving the combination of all four Houses, Luna Lovegood will play a major role in book 7.
2. Harry's ability to speak with snakes (which he shares with Voldemort) was important in early part of book 1, very important in book 2, and has been mostly ignored since then. I predict that it will be important in book 7, most likely with Nagini.
3. In the penultimate scene of movie 5, Luna (searching for her lost shoes), talks with Harry about Sirius's death, and explains that important things which we have lost often come back to us, although in unexpected ways. She immediately finds her shoes, tied to a rafter. In a movie that had to make tough decisions about condensing a 900 page book (with Rowling supervising the screenplay and every detail of the movie--including where objects are placed), I think that the inclusion of this seemingly trivial scene points us very strongly to Sirius meeting Harry again, somehow.