As mentioned in the book I’m sure that Regulus Black is R.A.B., I’m sure the locket mentioned on OOP is the real Horcrux, and I’m pretty sure Regulus stole the real Horcrux with the help of Kreacher.  Despite that, we’re left with another question; one that requires us to try and read the intent of J.K. Rowling:  why was the Horcrux fake?  Or, putting it differently, “how is the story advanced by the locket being fake instead of real?”

 
        All of the following is pure speculation but I suspect the following:  The Horcrux in the cave was fake, and the Horcrux at 12 Grimmauld Place was real, for one reason only:  to get Harry back to Sirius’ old house after the death of Dumbledore.  That much is fairly self-evident.  The next question is, of course, why?  Why bring Harry back to the house?  Well, going back to the house brings us back to Kreacher, and also, I suspect, to Regulus Black.

        Remember, that we don’t know if Regulus is alive or dead, and I suspect he may be alive because of the following:

MA: And perhaps, being Sirius’ brother, he had another mirror -

JKR: [drums fingers on soda can]

MA: Does he have the other mirror, or Sirius’ mirror -

JKR: I have no comment at all on that mirror. That mirror is not on the table. [Laughter from all; Jo's is maniacal.]

MA: Let the record note that she has drummed her fingers on her Coke can in a very Mr. Burns-like way.

                                           (Leaky Cauldron/Mugglenet interview, 2005)

 
        So, JKR won’t talk about the two-way mirror?  Well, there’s a giant red flag.  And when you look at how and where that mirror appears in OOP, the flag only gets redder. 

The two-way mirror JKR and Melissa Anelli are discussing appears in OOP only twice.  It first shows up, wrapped as a gift, on page 523 when Sirius gives it to Harry.  It then disappears from the story for 334 pages, until page 857, when Harry finds it at the bottom of his trunk.  Now, does it make sense to you that Harry would forget a mysterious Christmas present from Sirius for an entire school term? No, of course it doesn’t. And the only thing we learn about the mirror is that it’s old, and dirty, and Harry can’t use it to talk to Sirius. If that’s the case, then why was it introduced into the story at all?  It’s got to be in there for a reason, right?

Now, from the extract of the interview above we know (not suspect, but know) that the mirror will be back.  And if it is, and if, as is certain, it can’t be used to talk to Sirius, I have to agree with Melissa and guess Harry will use it to talk to Regulus, or communicate with someone else intimately associated with the Black family.  The fake Horcrux, Kreacher, Regulus, the real Horcrux, 12 Grimmauld Place, and the two-way mirror are all part of the same whole.