Fan site MuggleNet claims two separate readers asked Harry Potter author JK Rowling at the Edinburgh International Book Festival in August whether Rubeus Hagrid was the next book's title character, or the half-blood prince.
Rowling allowed attendees to ask her questions while she signed autographs during her appearance, and according to these lucky fans, she answered "no" to both, when asked independently.
This should, of course, be treated as rumor, and it's not likely we'll get confirmation unless Rowling decides to make a statement on her official site. The author is currently writing the sixth book in her enormously successful series, Harry Potter and the Half-Blood Prince. A release date has not yet been announced.