I THINK I KNOW WHO R.A.B from the Half Blood Prince is! Haha I'm sure about a million people already came up with this, I'm a few years behind!

Anyway, so I was thinking about Harry Potter and the Half Blood Prince last night and you probably already know what just dawned on me but I think that RAB is Regulus Black, I'm not sure what what the A stands for but I'm pretty sure that it's Sirius's brother. In the note that is left in the locket, Voldemort was addressed as "the Dark Lord" which is how death eaters address him. Regulus was a death eater. It also says in the note that he faced death in removing the horcrux, and he did die (killed by either Voldemort or on Voldemort's orders.) So I think that he took the locket, and took it back to Grimmauld Place and left it there. Years later when Harry and Hermione and the Weasleys were cleaning it out they found a locket that none of them could open (OotP pg. 116?) I think that that is the horcrux. Of course , the question is now, why wasn't it destroyed? And where is it now?

I think Kreacher probably has it. He may have slipped it to the Malfoys on his little excursions but I don't know if he knew what the horcrux was, so there's a chance he still has it, since he collected shiny things.

AND THAT IS MY THEORY.

It all makes sense, it all fits together...I'm excited to know if I'm right!!