I also came to the conclusion that Regulus Black, Sirius`s brother, is the mystery RAB, and there`s a lot of evidence to support that.
Firstly, the note was addressed specifically to "The Dark Lord". That would suggest that whoever wrote it was a Death Eater, sonce Harry points out to Snape that only Death Eaters call Voldemort that. Secondly, Regulus is the only member of Sirius`s family to have been mentioned but not to have featured in the books, and JK seems to have gone out of her way to remind us that he used be a Death Eater, but tried to back out. Sirius says he got scared and tried to run, but as we know, Sirius has been out of contact with his family for years. Apart from that, the initials obviously fit.
Now, onto the matter of Snape and Dumbledore. Dumbledore most likely is dead, never to return, with the possible exception of forcing his echo out of Snape`s wand. As for Dumbledore begging for his life, Dumbledore probably wasn`t quite in his right mind at the time, due to the effects of the potion, which, as demonstrated in the cave, causes intense fear and pain. What interests me most about that scene is that every other Death Eater back away to clear a path for Snape, as if they were scared. Now, why would Snape kill Dumbledore? I can see several possibilities in this, and there are probably several more I haven`t seen.
Firstly, Snape is alligned to Voldemort and has been plotting this from day 1. I consider this one unlikely, since Snape protected Harry out in the grounds.
Secondly, the whole thing was framed and Dumbledore is not dead. This one is, if anything, even more unlikely.
Thirdly, Snape was forced to kill Dumbledore by the Unbreakable Vow he swore. Quite possibly the vow does not cause the bearer to die die if broken, but is exactly what it sounds like-unbreakable.
Fourthly, Dumbledore set his own death up, possibly with Snape`s knowledge. At first, this one seems unlikely in the extreme, but consider: Dumbledore makes a point that one of the most powerful weapons Harry has is his absolute determination, caused by his hatred of Voldemort. The reason for that is that Voldemort has been responsible for the deaths of his parents and Sirius. The only thing that could make him hate Voldemort more and thus make him more determined and more dangerous (barring the death of Ron, Hermione, Ginny, etc) is the death of Dumbledore himself. JK also makes the point that Dumbledore is unafraid of the sacrifice for the right cause ("The loss of hand does not seem an unreasonable exchange for a seventh of Voldemort`s soul") and that he feels that death in not such a great fear. ("To the well organised mind, death is but the next great adventure", "Once again, Voldemort fails to grasp that there are worse things than physical pain.") how about the sacrifice of his own life to give Harry the power to destroy Voldemort once and for all?
Finally, the idea of Harry`s scar as a Horcrux. I hadn`t considered this before, but it makes a lot of sense. There are a lot of similarites between Voldemort and Harry, as pointed out throughout the books. Also, as Dumbledore points out, Harry was likely to be the sacrifice needed to make Voldemort`s seventh and final Horcrux. If something were to go wrong with that process, the result could well be the transferring of a seventh of Voldemort`s soul to Harry. On the other hand, there is evidence against this. The most obvious one is that Voldemort has tried to kill Harry several times. The three possible explanations are that Voldemort is willing to sacrifice a seventh of his soul to destroy Harry (unlikely, Voldemort seems to be of an "everything or nothing" mindset, although he did put the diary at risk to reopen the Chamber of Secrets), that Harry is not the final Horcrux, or that Voldemort doesn`t know he transferred a part of his soul. However, even if Harry is the final Horcrux, that does not write out the possibility of an 8th. If Voldemort is unaware that he made a Harry a Horcrux or if he intends to destroy Harry regardless, he would want a 7th or to replace the lost one.
In closing, did anyone else think the end, with Harry breaking up with Ginny at the funeral of the man who tried to replace his father, was ripped stright out of Spider-man?