Um, slightly strange news this morning. J.K. Rowling has written a new book, but it isn't for sale.
She has written several copies of The Tales of Beedle the Bard (mentioned in the 7th book) but isn't publishing it. One copy will be available to the public by auction (with the proceeds going to charity). There's more information here at The Leaky Cauldron.
Incidentally, there are all kinds of new fun things on J.K. Rowling's website... this post at The Leaky Cauldron tells you how the new hidden secrets, and this one explains some of the cool new features.
Update: The book was sold at auction on December 12, 2007 and was purchased by Amazon.com.
There are more details about in this post.