1. One thing I'm completely sure about is that REB is Regulus Black, Sirius' brother. The missing horcrux locket was among the crap that the Order of the Phoenix threw away when they were cleaning out the Black house. Go re-read that chapter in OOTP if you don't believe me. There's explicit mention of a mysterious locket that nobody can open.
2. I'm fairly certain that Snape is still a good guy, and that when he attacked Dumbledore it was under Dumbledore's orders.
Stuff I think is very likely:
3. I think Malfoy is also good, and was also acting under Dumbledore's orders the whole time. (Or at least was indirectly through Dumbledore telling Snape what to tell him.)
4. When Snape took the blood oath promising to help Malfoy complete his quest, the quest he had in mind was escaping Voldemort's clutches, *not* killing Dumbledore. (Now that I think about it, it's possible Malfoy's mother may have known this. I'll have to re-read that chapter to see if it was a fake-out, and if Snape and Mrs. Malfoy were on the same page but a different page than the readers and the other woman present.)
5. I don't think Dumbledore is really dead. Book 6 took careful pains to establish the fact that a skilled wizard doesn't have to say a spell name to cast in, and in fact can say one spell name while casting a completely different one. The other times we've seen an avada kedavra spell, the victim simply keels over dead. But Dumbledore was expelled backwards, which is much more like the results of an expelliarmus spell. Immediately afterward his body is described as peaceful, almost like it's sleeping. And we never see his body again. His casket disappears in a puff of magic, conveniently erasing any evidence. I think Snape and Dumbledore wanted the world (and especially Harry) to think Dumbledore was dead, but he'll show up at the end of book 7.
Stuff I think is somewhat likely:
6. I think Neville will kill Voldemort, not Harry. The prophecy said it could be either of them, and the explanation that "Oh, it must be Harry" seemed a bit hand-wavy. Why would Rowling even say the prophecy could be either one if it wasn't going to pay off somehow?
7. I think Ron is going to die. No real evidence to back this up. Just a feeling I've had since Book 1. Also there are striking similarities between Harry, Hermione, and Draco to Dumbledore, McGonagal, and Snape, but there's no real counterpart to Ron.
My crazy wild-ass idea:
This is also something I've had in mind since Book 1. I don't think it's too likely, but since it's something nobody else is saying, I'll throw it out there so if I'm right I can tell everyone "I told you so."
8. I think there's a possibility that Harry may end up somehow going back in time 100 years or so, at which point he will change his name to Dumbledore. In other words, Harry and Dumbledore are the same person. That would explain why Dumbledore always believes Harry when he needs to and disbelieves Harry when Harry needs to do stuff on his own, why he always shows up at the right time and only the right time, and why he never seems to be particularly worried that Harry will die. Because he's already lived through it, and knows how it turns out.
Anyone else have predictions?