Last Minute Speculation
I haven't read HBP yet, so this is just your average harebrained theory and speculation post. No real spoilers!
I'm putting it under cut however, since it contains material of a nature that is touchy nowadays. Yes. I am going to speculate on who's the HBP, but: I have not yet read the book, so it is all based on clues everyone has had the last few weeks.
I've been thinking...
So, who is the Half-blood Prince?
In the following, I will not consider the possibilities of him being not a person or a new character, since that would be a very short answer indeed.
I think we can assume safely, that the HBP is
a) a Half-blood,
b) male.
We know for sure, since JKR stated it, that the HBP is neither Harry Potter nor Tom Riddle/Lord Voldemort. There are quite some Half-blood people at Hogwarts, but none of those strikes me as particularly likely. Then, there are the purebloods, such as Longbottom, Malfoy, etc... even Sirius Black, should JKR feel the need to let him return.
And then there are those of whom everybody just assumes they are purebloods, since it would be an obvious conclusion. Death Eaters, Slytherins, etc...
But there is in fact one Slytherin, who also happens to bear the Dark Mark and whose ancestry is far from ascertained. I know you are probably thinking, 'oh, noo, not *again*! Will she ever shut up about him?', but just hear me out, please.
Yes, I am talking about Snape.
I've often heard people say, oh, but he's a pureblood. But let me tell you, there is actually no reference in the books whatsoever that would prove it as a fact. What is more, JKR has been particularly evasive about Snape's pedigree.
When asked, she stated that since he is (or used to be) a Death Eater, it can be assumed that he is not Muggle-born. Other than that, she only said that "his ancestry has been hinted at". So, what conclusions can be drawn from that?
For one, it appears that his ancestry is relevant in some way. If it were not, would she walk on eggshells like that and go out of her way to not answer the question? I seriously doubt it.
For another, I don't think he is a pureblood, since if he were, that would be the obvious conclusion and we could just let it be over and done with, no need for vague answers and enigmatic behaviour on JKR's part.
It would therefore appear, that Snape is, in fact, a Half-blood.
I have also found a hint pointing towards Snape not being a pureblood, or rather, since it has never been talked about it, a non-hint. The crucial point being exactly that no one has talked about it.
I think we all remember when the late Sirius Black shows Harry the Black family tree, stating that "all the pureblood families are interrelated". They end up talking about the Weasleys and the Malfoys, Tonks, the Lestranges and quite a bunch of other people. Harry is particularly apalled about the connection to the Malfoy family. Wouldn't you think, with all the UST tension between Snape and him, he would have at least made a remark about Sirius being related to Snape? I think he would have.
Which suggests that there is in fact no such relative on the Black Family Tree. But if all the pureblood families are interrelated and Snape is not related to them, it would follow by implication, that Snape is not a pureblood.
We don't know much about Snape's parents at all, or his family in general. What we do get is a glimpse of an early childhood memory, showing a "hook-nosed man" towering over a dark-haired woman and shouting in a rather poor environment. I do not believe that poverty is an argument against being a pureblood; just look at the Weasleys, and there you have proof of the contrary. But assuming, just for a moment, that those people are Snape's parents (which is also never confirmed), and that one of Snape's parents is a muggle, takes that scene to a whole new level of significance.
What if his father is the muggle? Wouldn't you think that seeing him behaving like that towards his mother (who is a witch) would make young Severus Snape a bit angry? Maybe even angry enough to make certain alliances, especially if that was not the only incident of that kind? But I digress.
Snape as a Half-blood also offers an explanation for the animosity between him and Sirius Black. We recall that Black and Snape hated each other on sight. Sirius seem to think (whether it's true or not may be as it is) that Snape comes to Hogwarts as one who knows more about the Dark Arts than any seventh year. In terms of eleven-year old boys, that is equivalent to Dark Wizard.
Sirius, who was born into a tradition of Dark Wizards, despises those and does his very best to distance himself from them. Mark that Sirius probably knows right from the beginning that Snape is a half-blood or "less". Seeing that, it is likely his proximity to the Dark Arts irks Sirius all the more: Snape seems to have chosen to be a Dark Wizard, he has betrayed his muggle relatives.
On the other hand, if what I assumed about that memory scene above is true, Snape would see Sirius as the same kind of traitor: socialising with Muggle-borns and such, although muggles behave violently against wizardfolk (mind that we are still talking eleven year old boys!), and Sirius who has the best predisposition to work against that, throws it all away.
I think even if it is not proven that Snape is a Half-blood, there are at least some arguments pointing in that direction.
The *Prince* issue however, is a bit trickier. Aristocratic titles are never discussed in the books, so we have to look elsewhere for clues. This is where it get very very vague.
Let's take a look at what we have: For some time now, the editors have been publishing the book covers for Harry Potter and the Half-blood Prince (which are unfortunately currently unavailable on mugglenet).
The UK adult cover for instance shows a very battered looking book by the title of "Advanced Potion-making". I don't believe this is one of Harry's books, since a) it isn't even certain he will be in NEWT Potions, and b) he could usually afford new and shiny books. Therefore, I think this cover image is about Snape, stating that somehow Snape will be significant in HBP.
The US front cover shows Harry and Dumbledore waving a wand over some sort of glowing bowl. That bowl, in my opinion is most likely a Pensieve. Given what events involved Pensieves so far, this could also be an allusion to Snape's importance in the HBP.
The UK children's edition front cover shows Harry Potter, Dumbledore, and a lot of fire-y stuff. Nothing to be going with there.
The back cover of that edition however is a bit more interesting. It shows (among others) a pair of hands grasping each other, the younger of which is by popular belief Harry's (and I don't see who else is more likely), and the older of which... at least leaves room for speculation.
The supposed age of the arm owner could be something 40-ish, which is approximately Snape's age (36-37 at the time of the HBP). Furthermore, the arm is clad in a black sleeve, which, as we all know, is the colour of Snape's usual attire.
And, as far as official publications go, there is also the artwork which could be found in the bloomsbury screensaver. That one is very interesting, since it shows the same pair of hands in the background, and in front of it there's a golden ring with a cracked black stone.
The stone in itself is interesting, since it appears to be designed with an illustration of a shield/crest. The actual image on it is very difficult to discern, but if you remove the crack in the middle and put the parts together it could be showing a serpent. It is therefore possible that the ring is an artifact connected to Slytherin House, which, as we all know, happens to be Snape's house as well.
Another interesting detail is, as I believe, the shape of the crack in the stone: it is shaped like a lightning bolt (make of that what you wish, I only thought it deserved mentioning).
And last (but perhaps not at all least), there has been an interview with an official representative of the editors of HBP on US television, where the issue of who might be the HBP was discussed. The interviewer seemed to have a fondness of Snape, and asked whether maybe Snape was the HBP. The representative, upon hearing the question, appeared to be startled for a few moments, maybe even taken aback before he told the guy that he can't reveal any details.
Seen for themselves, none of these arguments would be plausible enough to justify Snape as the Half-blood Price. However, if one looks at the overall picture, it seems possible, maybe even probable. This is why I believe Snape is the Half-blood Prince.
That's it, folks! As of now, I'm going on hiatus. I will not read flist. I'll return when I've read the book. Which is going to be tomorrow night... So long, and thanks for all the fish ... XD