She is not done with the book, and there is no estimated date of publication. She does say that the “Half Blood Prince” isn’t Harry or Voldemort.
(Half-blood almost has to refer to what derisively is called a “mudblood” or someone who is only half-wizard.)
I suspect it won’t be a minor character
For those who wish to start making guesses…JK Rowling has stated that she considered the title for the second book, but decided crucial bits of information were better revealed in book six. Suggesting we knew the HBP in the second book, we just didn’t know he was an HBP.
Here’s my analysis of the possibilities:
Neville – I think he is a possibility. We know his father was a wizard, and came from a long line of respected wizards. He now lives with his grandmother I believe. I’m not sure we know about his mother.
Weasleys – not a possibility, we know about both their parents.
Draco – not a possibility, we know about both his parents
Adults (almost all of them are possibilities)
Snape – interesting possibility since most of Slytherin hates ‘mudbloods’
Hagrid – definite possibility
Sirius – not introduced by book two, so unlikely
Dumbledore – possibility
Those are probably are the most likely guesses. And I would pick Neville if I were betting. Just because it appears he’s becoming more and more important as the books progress. (I even think it is possible, if Rowling is a brave woman, that Neville might end up being the one to kill Voldemort in the end, and not Harry. But legions of kids would probably hate her forever if she did that, which is why she’d have to be very brave. But if she turns him into a prince in book six, it might be more acceptable to the legions. These are just guesses. I know nothing.)