Ok, maybe I'm an idiot, but I'm just...stunned. I just didn't think Dumbledore would die in this book. And to have it be Snape...that..that...I just don't even know what to say.
As for Regulus Black, remember when Sirius was discussing him in OotP ? He said that his parents supported Voldemort, but that they'd backed away a bit when they realized how far he had gone into the Dark Arts. My guess is, Regulus also had a similar epiphany, and was killed after first joining, then trying to back out of the Death Eaters.
So, yes, I believe that it's
very plausible that Regulus is "R.A.B."
Just for the record, I'm
really glad that this thread was here tonight. I really wanted to share this book with someone, but DW hasn't even started it yet, so I was kinda stuck, particularly after reading that ending. Glad there are quite a few marathon readers out there to share it with.