According to a tipster, notice was sent to book publishers that warned them to "be prepared to print copies of the next HP book" by that date. The site's analysis theorizes that since Order of the Phoenix was released on June 21, or summer solstace, it may be that author JK Rowling is attempting an autumn equinox, or September 22, release for book 6.
Keep in mind a Smith Barney financial advisor who follows American Potter publisher Scholastic said in February that he doesn't expect the next installment until summer 2005 at the earliest, but more likely the summer of 2006.
So probably not. But that's what I want for my birthday. Give it to me.