I know there aren't very many facts now, but that will change as time progresses...

• J.K. Rowling says that not even her friends and family know what the sixth book will be called.
• "Six will not need to be that long. I had to move them around a lot in there, there's a lot of to-ing and fro-ing in there." (From the BBC Interview with Jeremy Paxman)
• More deaths are coming and the books are getting darker.
• J.K. Rowling says that death is perhaps the biggest theme in her books. She says that she's writing about Voldemort ("essentially a psychopath") and she wants to show the true evil of taking human life. (These are quotes from the show "J.K. Rowling: Harry Potter and Me.")
• A character manages to learn magic late in life. (Aunt Petunia?)