“The locket they had managed to steal so many hours before had fallen out of Dumbledore’s pocket. It had opened, perhaps due to the force with which it hit the ground. And although he could not fell more that shock or horror or sadness than he felt already, Harry knew, as he picked it up, that there was something wrong…
He turned the locket over in his hands. This was neither as large as the locket he remembered seeing in the Pensieve, nor were there any markings upon it, no sign of the ornate S that was supposed to be Slytherin’s mark. Moreover, there was nothing inside but for a scrap of folded parchment wedged tightly into the place where a portrait should have been.
Automatically without really thinking about what he was doing Harry pulled out the fragment of parchment, opened it, and read by the light of the man wands that had now been lit behind him:

To the Dark Lord
I know I will be dead long before you read this
Bu I want you to know that it was I who discovered your secret.
U have stolen the real Horcrux and intend to destroy it as soon as I can.
I face death in the hope that when you meet your match,
You will be mortal once more.
R.A.B.

Harry neither knew nor cared what the message meant. Only one thing mattered: This was not a Horcrux. Dumbledore had weakened himself by drinking that terrible potion for nothing. Harry crumpled the parchment in his hand, and his eyes burned with tears as behind him Fang began to howl.”

-Harry Potter and the Half Blood Prince, Chapter Twenty-Eight, pp. 609-610

We are left in wonderment as to where this Horcrux could be. ‘Who is R.A.B.?’ and more importantly, ‘Where is this Horcrux?’
A couple of weeks ago I was listening to the fifth book as narrated by Jim Dale. When a seemingly trivial part of the story intrigued me. To give you a little background, Harry and company are at the Order of the Phoenix headquarters, #12 Grimmauld Place, attempting to clean the vast garble of dark instruments and trinkets left behind by the House of Black:

“They found an unpleasant-looking silver instrument, something like a many-legged pair of tweezers, which scuttled up Harry’s arm like a spider when he picked it up, and attempted to puncture his skin; Sirius seized it and smashed it with a heavy book entitled Nature’s Nobility: A Wizarding Geneaology. There was a musical box that emitted a faintly sinister, tinkling tune when wound, and they all found themselves becoming curiously weak and sleepy until Ginny had the sense to slam the lid shut; also a heavy locket that none of them could open, a number of ancient seals and, in a dusty box, an Order of Merlin, First Class, that had been awarded to Sirius’s grandfather for “Services to the Ministry.”

-Harry Potter and the Order of the Phoenix, Chapter Six, p. 116

If one were to take the theory that R.A.B. stood for Regulus A. Black, then this idea that I am proposing fits quite well. It would make sense from the story to have such an item located in such a dark house.
My own inclination as to where this horcrux is the possibility that Kreacher, the Black house elf, stole it out of the bag before it was thrown out.
You have not heard the last of #12 Grimmauld Place.
If one read further into the fifth book, Sirius goes on to explain how Regulus Black died:

“’Oh no,’ said Sirius. ‘No, he was murdered by Voldermort. Or on Voldermort’s orders, more likely, I doubt Regulus was ever important enough to be killed by Voldermort in person. From what I found out after he died, he got in so far, then panicked about what he was being asked to do and tried to back out. Well, you don’t just hand in your resignation to Voldermort. It’s a lifetime of service or death.’”

-Harry Potter and the Order of the Phoenix, Chapter Six, p. 112

The issue of Voldermort’s horcruxes, his way of achieving immortality, would be something to be kept quiet; something known only to his most trusted supporters. Lucius Malfoy, for instance was trusted with the Tom Riddle diary Horcrux given to Ginny in the second Harry Potter book, later destroyed by Harry.
Although this locket horcrux was not given to Regulus Black, something must have been asked of him in relation to this horcrux or the idea of horcruxes. He panicked and tried to back out, but was killed by Voldermort.

My ideas fit in so well with the literary style of J.K. Rowling. She has a tendency to draw upon seemingly trivial details and making them pivotal points of her story. Who would have guessed that something as normal and unimportant as cleaning a house could uncover one of the greatest cliffhangers of this saga? That’s my two cents…and then some.
What do you think?

The Potter madness never ends: GOF is so soon!

It would be the weirdest thing ever if Harry died in the seventh. So, yeah, he's got to find the horcrux(es) pretty quickly.

Honestly, though, the horcrux can't be too far off. We've only got one book left, eh? And everyone knows Harry is not going to die in the seventh, so he's obviously going to find it, probably destroy it, and kill Voldemort. Fairly straight-forward really. w00t! I just became re-potterized. Off to mugglenet!

I agree, red. That's what I've thought since I read it. It's probably likely that Kreacher has it, but Sirius could also have thrown it out or Mundungus could have pawned it off.

You better!

I'm so not in the HP-verse...that none of anything they said made any sense. Except ICE.

I read up to Book Four as they were released..but I stopped before Book Five. I'll catch up before its corresponding movie is released.

In the next book Dumbledore comes back from the grave as a zombie and eats Ron. Then Hermione slips on a banana peel and falls down some stairs.

hmm.... very interesting. yeah i think your on to something too. nice work. but does it say anywhere in the books what sirius's or regulas's middle name might be? or any other names that start with the letter A that are also related to the Black Family?

Wow. I think you may be onto something there. Good catch.

Hm. My theory is that Regulus relised what he'd gotten himself into and what it all added up to and freaked. So he decided to try to take care of part of the problem and destroy the locket. However, the death eaters caught word of this and killed Regulus after he destroyed it. Pretty much the same theory as yours, Red, just a bit of a different twist on the whole Regulus thing.

We fans have just gotten smarter over the eight years (aprx.) we've been reading the books.

I don't know, I'd hardly declare the locket in Book 5 "obvious." I think Rowling's readers have just finally gotten one step ahead of her.

No offense, Red but I (along with others) have been saying this since we read book 6. I think that this scenario makes sense, but it may be too obvious. Rowlings is never obvious, and perhaps RAB is a character whom we don't even know yet.