JK Rowling’s Official Website has been given a totally new design and tells visitors that:

J.K. Rowling is to publish a new novel for adults.
Further details will be announced later in the year.
J.K. Rowling’s official website is currently being refreshed and redesigned, and will be relaunched later this Spring.

You can sign up for email updates and announcements. Additionally, The Blair Partnership has released a Press Release with some more information on the book’s release that quotes JKR as saying:

“Although I’ve enjoyed writing it every bit as much, my next book will be very different to the Harry Potter series, which has been published so brilliantly by Bloomsbury and my other publishers around the world. The freedom to explore new territory is a gift that Harry’s success has brought me, and with that new territory it seemed a logical progression to have a new publisher. I am delighted to have a second publishing home in Little, Brown, and a publishing team that will be a great partner in this new phase of my writing life”

The book rights were bought by Little, Brown and Company in the UK and US and will be released in print and electronic form simultaneously.
Finally, The Associated Press has a new report online with some more information.

Rowling’s agent, Neil Blair, would not disclose financial details of the deal but said Thursday there had been no auction.

“As her new book is for a different audience, and marks a new literary direction for her, it made sense to separate the two and for her new book to be launched by a different publisher,” said Blair.

In a statement, Bloomsbury said its 15-year relationship with Rowling “remains stronger than ever” — and new editions of the seven Potter novels were on the way.