I've heard a few people bring up the idea, but I'm kind of surprised that Dumbledore's death doesn't look like a mercy killing to everybody. That hit me as soon as Dumbledore started begging Harry to kill him after drinking the potion (which seemed to be something important and full of clues, the way he was screaming and cowering -- essence of dementor juice or something? If so, Dumbledore's worst memory seems to be of having let innocent people in his protection suffer); even before he sees the Dark Mark at the Three Broomsticks, he's begging for Snape and refusing Madame Pomfrey; that coupled with "it wasn't a health drink" makes me think that he knew he was dying, and just wanted Snape to finish the job so either he wouldn't keep suffering uselessly or wouldn't transform into something horrible as a result of drinking the potion. I even suspect that he might have scouted out the basin before leading Harry there, and maybe Snape's telling him about the Unbreakable Vow had something to do with his self-sacrifice, but I think there must be more to it than that.

Also, I wasn't clever enough to think of it, but R.A.B. = Regulus A* Black, yes? Are there any other possible contenders?

I don't think there can be a good equivalent of a Horcrux; splitting one's soul seemed emphatically to be a very very bad thing. Though I do see certain parallels in the idea of the Patronus.

Also, I so TOTALLY called Snape being the Half-Blood prince as soon as I heard that the annotations in the Potions book were in girly handwriting. Ha!