I can't be bothered to read through all these posts so I'll just post my views on the book's mysteries now, excuse me if I do end up repeating crap!
Firstly, as I keep repeating to everyone, this is primarily a children's book, which in almost all cases will have happy endings. In fact almost all "epics" of this nature (including Lord of the Rings which was an adult book) end happily, despite the seemingly decreasing hope of such an ending throughout the body of the tale! I would be extremely suprised if Rowling did not end this whole tale happily.
Using this idea to support my theories, I can discuss the death of characters. In epics of this sort, semi-major characters on the good side can be killed off, but not major characters. Cederic Diggory is a semi-major character (mentioned more than once and has some talking parts etc)....he will not be coming back! Black and DD are major characters. They WILL be coming back....in one form or another! I can not guarentee that they will return in their full bodily form but who knows what magic Rowling will invent to achieve this purpose!
The character most likely to return in full bodily form is Black, as you should all know, no character is "dead" until you've actually seen their dead body on the floor! We didn't even see Black die, he just fell through some stupid door....as soon as that happened I felt confident that Rowling planned to bring him back later on miraculously!
DD unfortunately appears to have died though because indeed we have seen his dead body! This does not mean he is gone forever, there are some important points worth noting about the events surrounding his death!
Firstly, we all know that Rowling isn't the type of author to leave actions unexplained, and many actions up in the tower have so far been left unexplained - actions explainable only by DD. The most striking of examples is DD's freezing of HP. We can all speculate as to why he did this (he didn't want HP getting involved in the following discussion or series of events - which I believe to have been rehersed but I discuss that below), but ultimately Rowling ALWAYS tells us explicitly later on why these actions occured. DD has to come back at least to explain actions such as this one!
Secondly, as you have all pointed out, Snape HAD to help Draco to avoid being killed himself, and DD knew this. And we also know that DD was aware that Snape would only last one year as the DADA teacher. DD surely must have had some sort of rehersed series of events to mean that Snape would leave hogwarts (hopefully to go deeper into the dark side as a spy). When DD was "pleading" with Snape, we can question what DD was pleading for! It is wrong to assume he was pleading for Snape not to kill him because 1. If snape didn't kill him, nor did Draco, then one of the Death Eaters would have. 2. DD must have planned that snape would leave hogwarts...he would not have made him DADA teacher without such plans, which would have otherwise been sentancing Snape to death! I reckon it was DD's plan all along that Snape survived and left Hogwarts "supposedly" with the dark side!
I am confident that these series of events had been pre-arranged between Snape and DD. There are a couple of other interesting points I thought worth mentioning, although I do not know whether they are truly relevant or not!
Firstly, Rowling as a writer often introduces a concept which she will unknowingly later on use in an important series of events. The important concept introduced in this book was the idea of "non-spoken spells". Combining this with the seemingly "unique" behaviour of DD's body when struck with the supposedly "avada kedavra" curse may arouse suspicion. In previous books, the AK curse just caused the victim to die on the spot, whereas DD's body was blasted up into the air and appeared to hang in mid air for a moment! If we combine this with the introduction of non-spoken spells, could it be a possibility that Snape just SAID "avada kedavra" but in fact cast a different, non-lethal spell?!
Finally, as a last worthy point to note (well I can't think of any others at the moment), there is a striking "space-time region" of uncertainty in that whole chapter. What is this region? It is the spossedly shear drop from the tower to the ground, and the time period is that between DD falling over the edge and people finding his body on the floor (and to a lesser extent, his Wand being thrown over the edge as well....it's whereabouts not being mentioned are indeed curious, there does not seem to have been a recovery of it - no mention of it being burried with the body or anything...suspicious!?). When DD's body was thrown over the edge of the tower, can we presume for that moment he was indeed STILL ALIVE. It is often a trick of authors to insert these regions of uncertainty......where ANYTHING could have happened but we assume only the worst! I can not think of any directly parallel example, but taking Gandalf's "death" in moria as an example....when he fell from the bridge, the reader assumes his death. Likewise with this situation, we have assumed DD's death because he fell from the tower and then 15 mins later we find his body on the floor. 15 mins is a long time, and with the disappearence of the wand I am somewhat suspicious of what appears to have taken place: it is far too big a whole for Rowling not to exploit...if she wanted DD to be truly dead, he would not have been thrown over the edge but instead very obviously killed on the spot (although then again perhaps it's just more "dramatic" to have him thrown from a tower...)
Finally, regarding R.A.B.....if those are indeed the true initials of a character, and the character has already been mentioned, then it would indeed be Regulus Black if anyone! Although I would be a little disappointed with Rowling if that was the case, it is somewhat unlike her to leave such an obvious clue. Some part of me feels she planted that to mislead us, and perhaps that initials are actually for some sort of nickname or actually mean something COMPLETELY different. It just seems all too obvious that it's Black....how many times before has Rowling guided us into what seems all too obvious before eventually showing us it was something else that was the truth! I would be sceptical at this stage!
Phew that's a lot, I don't know what you guys will make of that!?