Bloomsbury's not saying when precisely Harry Potter and the Deathly Hallows is out - but its last business statement holds a strong clue. In the first six months of '06 it generated almost no income at all. Bloomsbury promised however that it remained "confident of a satisfactory outcome” in 2007 when the City expects that earnings will be about £21 million. Only a new Potter book could deliver such a result.
In all probability it will be released on July 7, 2007 - a date with significance to numerologists. Deathly Hallows is the seventh and last book in the series and its release on 7.7.07 is a marketing dream. Potter fans also know that seven is a significant number to JK Rowling.
At this link there's more on the number and includes this: "In Half-Blood Prince, it was revealed that Lord Voldemort had made seven horcruxes (himself included). Quidditch teams have seven players each. Gryffindor Tower's entrance is on the seventh floor at Hogwarts."
Bear in mind that there are currently three more Harry Potter films yet to be released. On April 5, 2006, Warner Brothers announced that the fifth film, Harry Potter and the Order of the Phoenix, will be released in cinemas on July 13, 2007. It would make absolute sense to release Deathly Hallows at about the same time for maximum synergy.
Deathly Hallows is already No 1 on Amazon.co.uk thanks to pre-ordering.