I'm Sticking With 7/7/7
So I was listening to some Harry Potter Podcasts (Pottercast & Mugglecast, if you care to know) and the general consensus is that Book 7 just can't come out on 7/7/07 because the HP:Order of the Phoenix movie is slated for 7/13/07.
Yes, I realise that a bunch of people with podcasts think that it would be "stupid" and a "promotional nightmare" to bundle the two events so closely.
Personally I think that is exactly what WB/Bloomsbury Group has in mind. They WANT a juggernaut. They WANT Harry Potter to own July, 2007.
Why, exactly, would it be STOOPID, Melissa? Films have product tie-ins all the time. For instance, the Cars toys have been at McD's and Walgreens for at least 3 weeks ahead of the Pixar film. Books, on the other hand, often have to wait until they're officially backlist titles to get a good film tie-in.
The amount of pop-culture noise created by this joint release would be phenominal. Besides which, everyone in HPFandom is making boohoo noise about it "all coming to an end" after the release of book 7. (Another thing I disagree with....) Why not go out with a Grand Finale?
So, I'm sticking with my prediction of 7/7/07.