Who is "RAB"?

[Professor Lupin™]

"To the Dark Lord
I know I will be dead long before you read this
but I want you to know that it was I who discovered your secret.
I 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."

So who do you think R. A. B. is? One of the most popular theories is Regulus Black (Sirius' brother), and I have to say that I agree. Firstly, the message addresses Voldemort with "To the Dark Lord". From this we can gather that R. A. B. is a death eater, since only death eaters refer to Voldy as "the dark lord" (in Order of the Phoenix, Harry asks Snape why he calls him "the dark lord" and not "you-know-who" like everyone else). We know that Regulus Black was a death eater, as Sirius told us in Order of the Phoenix.

We get the impression from the message that the writer is dead, and Sirius tells us that his brother was killed by death eaters: He was murdered by Voldemort. Or on Voldemort’s orders, more likely, I doubt he was ever important enough to be killed by Voldemort in person...he got in so far, then panicked about what he was being asked to do and tried to back out." So Regulus was a death eater, who became disloyal and wanted to leave...perhaps even wanting to destroy Voldemort and his horcruxes? All fits so far.

The line "I face death in the hope that when you meet your match" also implies that the writer knows about the prophecy...he knows that a wizard is coming that can bring Voldy's downfall.."a match". We know that Regulus died 15 years before OoTP, and that Trelawney made the prophecy sixteen years before OoTP, therefore giving Regulus a year's knowledge of the prophecy before he died.

Here comes the best bit. Reading through Ootp in the chapter where Harry, Ron and Hermione are cleaning up in Grimmauld Place (home of Regulus) they find a "heavy locket that none of them could open." Could this be the locket horcrux? It seems a good place to hide it, as we know that Sirius' father protected the house very well. If this is the horcrux, then they have thrown it away. Or did Mundungus steal it? We catch him taking Black family silver, a locket may have also been appealing..

Supposing that R. A. B. is Regulus Black, he must of had an accomplice to get to the locket. We know that two people are needed from Harry's attempt: Dumbledore must drink the potion and Harry gets the locket. We also know that this accomplice must not have significant powers, as the boat will only take one fully grown wizard (which is why Harry is able to go with Dumbledore). Kreacher seems an ideal candidate for Regulus' accomplice - not counting as an adult wizard, and he is also bound to follow his masters' orders.

So, what do you think?

Lupin

[Professor Zephyr X. Nemo]

Great topic, I never really thought of the second person necessary until now. I don't think there's much doubt that R.A.B is Regulus, and the locket mentioned in the book is very probably Slytherin's. Why else even mention it if it didn't do anything special?
Regulus' use of Kreacher would make sense. Regulus could have ordered Kreacher to drink the potion without his refusal, as long as the drinker didn't have to be human and if drinking it wouldn't kill a house elf. If that's the case he could have ordered Kreacher to make him drink it instead. We never found out if drinking the potion would have killed Dumbledore in the long run, maybe drinking it was really the reason Regulus died?
Hard to say until book 7, as is whether Mundungus stole the locket... If he did manage to take it, it would probably have been sold back to Borgin & Burkes. I think there's mention of how much Borgin would like to have it back in his shop again, during the pensieve scene with young Riddle and the old woman. Just a thought.