Here is what she says about this title on her website:
Title of Book Six: The Truth
Well, the door opened at last and I showed you the title of book six - the genuine title, the title that will appear on the published book, the title I have been using in my head for ages and ages. Unfortunately, however, the door opened on the very same day the 'Pillar of Storgé' hoaxer struck, which left a lot of Harry Potter fans bemused as to whether I was having a joke at their expense by posting another fake title to 'teach hoaxers a lesson', something I certainly wouldn't do, as it would simply frustrate, confuse or annoy the 99.9% of you who aren't hoaxers! I tried to give a clear hint that the title behind the door was the real one by making the 'Toenail' joke as well (see 'Rumours'). But just to clear up matters once and for all…
Information you take directly from this site will be truthful and accurate (I might occasionally joke, but as time goes on, you'll learn to tell when I'm joking). Do not trust anybody else claiming to have found information on this site that you cannot access, however seemingly convincing the images they provide to support their story.
I never post information on the site that I do not want fans to read immediately. In other words, anybody claiming to have 'discovered' a message that wasn't due for release yet is lying. There was never anything meaningful behind the door until the 'Do Not Disturb' sign came off!
The 'Pillar of Storgé' was never my title, and I did not change it at the eleventh hour because I was 'found out' (I nearly fell off my chair giggling when I read this).
I was delighted to see that a hard core of super-bright fans knew that the real title was once, in the long distant past, a possibility for 'Chamber of Secrets', and from that deduced that it was genuine. Certain crucial pieces of information in book six were originally planned for 'Chamber of Secrets', but very early on (first draft of Chamber) I realised that this information's proper home was book six. I have said before now that 'Chamber' holds some very important clues to the ultimate end of the series. Not as many as six, obviously, but there is a link.
Anyway: if you continue to exercise patience, you will find that the Do Not Disturb Door opens again… and again… giving you further hints about book six. But as a little bonus, and compensation for having been messed around by Mr. or Ms. Storgé, I shall tell you one thing without making you shift any bricks at all: the HBP is neither Harry nor Voldemort. And that's all I'm saying on THAT subject until the book's published.
Speculation is running rampant throughout the fandom, to the delight of many—including me.
First of all, in trying to determine just who the HBP is, I feel that one person we can safely rule out is Tom Riddle. I know some of you are speculating that, since TR and LV seem to have somewhat separate identities in CoS, and because Dobby’s hint in CoS turned out to mean TR was behind the attacks, that we can’t rule him out. BUT…
1) The thing with Dobby worked because he was referring to LV as “He-Who-Must-Not-Be-Named.” As Dobby explains to Harry at the end of CoS: “Was giving you a clue. The Dark Lord, before he changed his name, could be freely named, you see?” Dobby’s clue wasn’t based on TR and LV being two different people—it was based on the fact that the name "Tom Riddle" can be spoken, whereas saying “Voldemort” is taboo. But JKR isn’t doing what Dobby did, because she doesn’t say the HBP isn’t He-Who-Must-Not-Be-Named, she says he isn’t Voldemort. In CoS, when JKR reveals the answers to the mystery, the big revelation is that Tom Marvolo Riddle=I Am Lord Voldemort. After this point, I believe she wants us to view them as the same person, as further evidenced by AD casually calling him “Tom” at the end of OotP.
ETA: Also, TR was already using the name Lord Voldemort as a teenager.
2) She put that statement about the HBP not being Harry or Voldemort at the end of a long and emphatic announcement about how she does not wish to trick fans on her website. I highly doubt she would follow that with a statement as tricky as this would be, if, in order to get at the truth, we must discern that she’s not treating LV and TR as the same person. Moreover, this statement came after the title had already been available on the website for about a day. Theories had already begun to crop up all over the internet as to who the HBP might be. And who were the two characters most commonly suggested? Harry Potter, and Tom Riddle or Voldemort. In theorizing, I didn’t see people making much, if any, distinction between the two. I saw the possible HBP being identified as “Voldemort”, and people going on to point out his role—that is, Tom Riddle’s role—in CoS as evidence. I can’t be sure what JKR’s sources are, but clearly she caught wind of the speculation—that the two most popular theories for who the HBP is identify him as either Harry or LV/TR. In her statement, she comes across to me as trying to help fans get at the truth by debunking the two theories that spread like wildfire in the wake of the new title. It's not really an appropriate time to throw a LV=/=TR trick at us. It seems to me that, as usual, fans are having a hard time taking “no” for an answer. No offense to those of you who still believe TR might be the HBP, but that’s just how it feels me.
3) As davidjoates pointed out in his comment to dr_c’s post on this subject, the tile of “prince” is based on lineage, so it is a characteristic that is unlikely to change, even if other things about a person, such as looks and personality, do. It is therefore unlikely that TR would be the Half Blood Prince, but LV would not.
Therefore, I am going to say, with about 99% certainty, that the HBP is not Tom Riddle.
Apart from that, I really don’t know what to think. Of the suggestions I’ve heard so far, the following candidates seem plausible to me:
Perhaps with time, and further hints from behind the door, the answer to this mystery will come into sharper focus. For now, I’m happy to enjoy all the speculation. :D
Last, a meme gacked from seviet, the results of which are not surprising:
I am a member of 1 clique of size 14