First and foremost, my perception of the Dumbledore pleading scene, is simply this..I do not think he was pleading. He was in obvious pain from the potion he drank while trying to recover the locket and it was either a "oh sh*t, i should have seen this coming." or "do it, Snape, quick, I'm hurting here." I still can't decide. I admit, after reading that scene I was out for Snape's blood. I wanted him more than I want Voldy dead. But after reading all the posts, it made me remember the arguement scene with Snape and DD. So it could have been an argument like "Snape, you have to kill me!" "NO WAY, MAN! there has got to be another way!" kind of thing. So that could make absolute sense. DD is the wisest, most powerful wizard. He knew very well that he would be stationed in a picture somewhere in the office of, I guess McGonagall's office now, and that he would be able to lend a hand from there, and knowing that as long as he was alive the paternal affection that he has toward Harry, he would never allow Harry to fight Voldy by himself, but he knew this HAS to happen. So he removed himself from the equation, saving Draco's soul, Snape's life, and keeping the prophecy intact, at the price of weakening the movement and the Order of the Phoenix. I mean you take away a heavy hitter like Dumbledore, the Order all of a sudden doesn't seem like a very impressive thing.
Onto the RAB question: Who is he? I think everyone has pretty much nailed it. It probably is Regulus. I read somewhere on the internet, take this for what it's worth, that his middlename was Amadeus. This was written in May of 2005 so it's not like someone came up with it post July 15th. Again, take it with a grain of salt but if it is, then there is your RAB, a man who has saw all the evils of Voldy up close and personal, decided he wasn't cut out for it, and wanted to thwart him before being killed. But did some damage on his way out. Unfortunately, Dumbledore and Harry now have that "Go f yourself, Voldy!" letter and Voldy will never see it now. (another theory of RAB would be Burke from the store in Knockturn Alley..the one that we learned employed Riddle right after Hogwarts. He would know why Voldemort wanted these trinkets he was seeking and probably spoke on many occasions about horcrux's being that Burke owned a dark magical object store. we may learn his first name eventually. )
The Horcrux's's's's: the locket, the Hufflepuff cup, Nagini the snake, the ring and the diary. What's left? something of either Ravenclaw's, or Gryffindor's. and the part that lives in Voldy himself (If I'm forgetting something let me know). So if they have to find the real locket as it has been relocated by RAB, and the cup..probably found and relocated as well by our pal RAB. the Diary and the ring are destroyed. Nagini and Voldy will be found together more than likely..at the end. So the question is what's the 6th Horcrux. I think it would be pointless if it was something of Ravenclaw's...not really pertinent to the story. Logic would have it that it would be a Gryffindor object. The sword is still safely in the office so it can't be that, so it got me thinking and searching for other opinions..the one I'm thinking, is maybe Dumbledore was a horcrux..knowing this, and then learning of the vow that Snape made, and Draco's assigment from Voldy, and the prophecy, Dumbledore knew he would have to go eventually, so why not now when he can die as he lived, nobley and for a noble cause? These extenuating circumstances (the vow, MAlfoy) expedited his departure. Snape knowing this, knew he would have to do it and their argument was again of the "no way, Dumbledore, there has to be a better way!" variety. That would leave just Nagini and Voldy to go. Assuming RAB destroyed the other 2 objects already..or Harry finds them in book 7. The other theory I read, and I would be impressed if Rowling had the guts to do this to, forget us the readers, but to herself, is make Harry the Gryffindor Horcrux. We already know that Voldy transferred some of his ability to Harry and "marked" him. Maybe he transferred part of his soul? Think about it, in OotP Harry could see thru Voldemort and see thru Nagini when Mr. Weasley was attacked. There is a bond there. Like Voldemort could see the groundskeeper in the hallway listening in on his and Wormtail's conversation in the old Riddle House in the beginning of GoF via Nagini. If it's a part of Voldemort..a horcrux..and harry can do the same thing...wouldn't it be logical??? But of course that would mean that the prophecy would be incorrect. One couldn't live while the other does..but they would both HAVE TO DIE for Voldy to truly be forever vanquished!! Would she dare? I dare her to do that. She's going to put herself and all of us into therapy!

)
Either way, I love the direction this book has taken. Awesome! I am mad at myself for reading this book this fast. I am already reading it over again.. I loved learning about Riddle, and I really feel like I lost a friend in Dumbledore. Great stuff.