My son is into Harry Potter, and has read (and re-read) all four books countless times. Now he's bugging me about when the next one's due to be published.
Does anyone have a clue?

Not for a while, I am afraid....
there are rumors (vigorously denied by JK Rowling) that she is
struggling with writer's block. Book 5 was supposed to be out this summer, now it is looking like early next year. Sigh. I, too, am waiting impatiently.

*drums fingers impatiently*

Yeh, originally, it was supposed to be in August I think...now they're thinking early 2003.

Actually the book was supposed to come out June 21,02 but now press is saying Febuary 8, 03

Whoa! So far away! I remember reading in the paper that it was going to come out before Xmas.

I herd that she wrote all the books and is slowly releaseing them so she can make more money, like with the lotr movies.

Just to confirm what has already been said there was an article
in the London Evening Standard about two months ago which went
along the lines of: single parent becomes multi-millionaire author
and loses the plot! The word at the time was that the author was
so busy enjoying her new role as a media celeb that hardly a word
had been wriiten. The summer of 2003 was being mentioned as the
release date but I would take any date given seriosly at present.