[DCA Fan]

I don't know about you, but when I finished HBP, the first words out of my mouth were "What the (explicative), JK Rowling?? Why did you have to leave such a huge cliffhanger???"

Now on to the theorizing....
I agree with most people that R.A.B. is Regalus Black, Sirius' brother. It makes fairly good sense, and can be connected with an earlier passage in the series.

I think Dumbledore did in fact die, and he was begging to Snape to kill him. The polyjuice theory just seems too implausible for me to believe. Dumbledore knew about the Unbreakable Vow, so to help Snape (lord knows why), he sacrificed himself. Harry was frozen so that he could not intervene and potentially get killed.