Unless someone else has thought of this...I haven't googled it, I want to bask in my own brilliance for a little while.
I think Dumbledore ENGINEERED HIS OWN DEATH. Yes, I do.
Follow me here.......
We know that Dumbledore asked Snape to kill him.
We know that the description of the curse Snape used is wrong.
We know that at the beginning of book 6, 5 potions were brewed. 4 by Slughorn, 1 by Harry, with the help of the Half-Blood Prince.
We know that the 4 potions Slughorn brewed were used in various ways throughout the book.
We know that Snape made an Unbreakable Vow with Bellatrix and Narcissa, supposedly to kill Dumbledore although that is not clear and I don't believe it is the case.
We know that Regulus Black stole the original Horcrux and replaced it with the fake. Kreacher has the real one and may or may not know what he really has.
We know that whatever the Unbreakable Vow referred to, Dumbledore knew about it.
So....
Dumbledore takes Harry on the quest for the Horcrux. In the process, Harry has to practically torture Dumbledore so that they can get it, but they do and just before Harry apparates them back to Hogwarts, Dumbledore VERY OBVIOUSLY "passes the torch" of fighting Voldemort over to Harry. He did that because he ALREADY KNEW he was going to die. He PLANNED it that way. Dumbledore has known since before Harry was born that either Harry (or Neville, but it turns out to be Harry) would have to kill Voldemort, or vice versa. Throughout the books, everyone standing between Harry and Voldy has been killed in one manner or another....everyone who loves Harry enough to try to protect him or get in the middle of the battle is dead, with the exceptions of the Weasleys and Hermione, but they know what has to happen.
The potion that doesn't obviously get used in the book, the one that Harry brews, is the Draught of Living Death. In the potions class, all the other potions are explained...what they do...but not that one. I think it's a pretty accurate description of what happens to Dumbledore after he drinks the potion in the cave, though. It strikes me as very odd that Regulus Black, who was by the time he got the real Horcrux a hunted man, would have had time to either brew or otherwise obtain a potion to refill the bowl.
I think Dumbledore did it. He found the cave, got to the bowl, filled it with the Draught of Living Death and later returns with Harry. He proves to Harry that in order to get what you want, you have to do repulsive things sometimes. After they leave the cave and apparate back to Hogsmead, Dumbledore is ADAMANT that he needs Snape. He needed Snape because he needed Snape to kill him. By his own engineering, he has weakened himself enough that Snape can do that. Snape wanted Dumbledore alive, he could NEVER have meant the Avada Kedavra enough for it to have a killing effect, so he had to use another spell to kill Dumbledore - a spell that would only be effective if Dumbledore was already incapacitated. He did that silently, and the AK out loud, so that no one would question his use of spell.
Voldemort never tried to kill Dumbledore before...why start now? Dumbledore is the only wizard he was every afraid of..that is said repeatedly. Why send a young boy (Draco) to try to do it, and why would Snape make an Unbreakable Vow to help him? That makes NO sense. For some reason that will become very clear in book 7, Dumbledore needed to remove himself...to spur Harry on, to make Harry angry enough to take on Voldemort....or for some other reason entirely. Snape would never willingly agree to kill Dumbledore, but he would also never sacrifice himself to keep Dumbledore alive. I don't think that is what the vow was about. Nevertheless, Snape does kill Dumbledore because Dumbledore needs him to. We already know there is something deep and secret between them...some reason for absolute trust that no one else is aware of. The ONLY reason Snape would do it is at Dumbledore's request. Nothing else would be good enough.