It popped into my head suddenly the other day, while I was cleaning the shower, that Snape might well be the one to kill Dumbledore (note how there is already zero doubt in my mind that somebody will kill Dumbledore in this book, whether it's Snape or not). If this is the case, I can see this being Harry and Snape's last and most terrible misunderstanding (note how there is zero doubt in my mind that it is a misunderstanding). Because unless JKR specifically tells us otherwise (and if she doesn't, I do not want to know, please and thank you, I will read it for myself), there's no way I'm going to believe that Dumbledore and Snape didn't hash the whole thing out together long ago.
If Snape's been acting as a double agent since the end of GoF, of course he would have to face the possibility of being one day put in a position where he'd have to kill Dumbledore as "proof" of his loyalty to Voldemort, and of course Dumbledore would have figured that out as well, and (knowing him) kindly told Severus not to worry about it, please, but to do whatever he felt was necessary for the sake of the Cause. Which, of course, Harry would never accept, and is guaranteed to react to IN ALL CAPS until and unless (I expect it will be merely "until", myself) he hears directly from Dumbledore himself (either in a letter or in some other magical form of communication) that All This Was Planned. So that might take us into Book Seven, because you have to know no way in the world is Snape going to say anything about it or betray his true feelings to Harry, being just as perverse and inscrutable about his personal business as Francis Crawford of Lymond ever was (and didn't some of his actions look awfully damning too?).
And now I'm babbling so I'll shut up -- but I just wanted this down as proof that if it should turn out to be true that Snape kills Dumbledore and it's not just a wacky idea of mine, I not only anticipated it but am not in the least perturbed by it, and it hasn't shaken my conviction that Snape is truly a good guy after all. Unless, of course, JKR unequivocally says that he's evil, but I really don't anticipate that from her, myself. She already dispensed with that in Book One: it's just Harry, who is a sweet boy but such a blockhead sometimes, who can't seem to figure out the point.
Oh, and if anybody tries to post real spoilers or what they have been led to believe are real spoilers in the comments: please don't. Like I said, this is an idea of my own, not something I want to see confirmed or denied by anything outside the text of HBP itself. Thank you.