By now, you've heard most of the arguments: Harry Potter has to live because his young fans wouldn't be able to handle his death. Harry Potter has to die so that no one else could continue J.K. Rowling's work (a legal impossibility, actually). Harry Potter has to stop by my place because he owes me money.
The problem with most of these arguments is that they ignore the internal logic of the Potter series. So let's go back to the books — because it's there we can discern a very real possibility that the world's favorite boy wizard may meet his maker (and we don't mean Rowling).
In "Half-Blood Prince," Dumbledore took Harry under his wing for private lessons. They dipped into the Pensieve together to view other people's memories so Harry could understand how Voldemort managed to stay alive so long. As you'll recall, Voldemort had split his soul a number of times and hidden the pieces inside Horcruxes. This was one of the darkest forms of Dark magic, created through murder, and not a lot was known about it. That's why one of the memories Harry and Dumbledore examined was that of Tom Riddle — the young Voldemort — as he sought out that information. Through the course of the lessons, Dumbledore was trying to get Harry to puzzle out what each memory revealed, what connected it to the ones he'd seen previously — from recognizing objects that could be used as Horcruxes as well as where they might be hidden. All of this information could have just been told to Harry, but no, Dumbledore preferred that Harry realize things slowly, just as Rowling was gently leading her readers along the same path, perhaps so we realize things just a moment before Harry does.
In "Half-Blood Prince," Dumbledore took Harry under his wing for private lessons. They dipped into the Pensieve together to view other people's memories so Harry could understand how Voldemort managed to stay alive so long. As you'll recall, Voldemort had split his soul a number of times and hidden the pieces inside Horcruxes. This was one of the darkest forms of Dark magic, created through murder, and not a lot was known about it. That's why one of the memories Harry and Dumbledore examined was that of Tom Riddle — the young Voldemort — as he sought out that information. Through the course of the lessons, Dumbledore was trying to get Harry to puzzle out what each memory revealed, what connected it to the ones he'd seen previously — from recognizing objects that could be used as Horcruxes to where they might be hidden. All of this information could just as easily have been told to Harry, but no, Dumbledore preferred that Harry work things out for himself — just as Rowling was gently leading her readers along the same path of discovery.
Dumbledore had been holding back information all along, of course. He hadn't told Harry why Voldemort tried to kill him for many years, reasoning that Harry was too young, he wasn't ready. And of course we weren't ready, either. But this information would explain why the killing curse Avada Kedavera didn't work on Harry, why some of Voldemort's powers had been transferred to Harry as a baby, and why Harry had the ability to see things not only from Voldemort's perspective, but also that of the Dark Lord's snake, Nagini.
The reason? Lord Voldemort had apparently (and accidentally) stored part of his soul in Harry. Harry is a Horcrux.
The sticky thing about the conventional wisdom about what happened to Harry and his parents — what it was that made him "The Boy Who Lived" — is that there were no witnesses (that we know of). "I am sure he was intending to make the final Horcrux with your death," Dumbledore tells Harry. Who's to say Voldemort wasn't trying to do both at once, killing the boy who had been prophesized to vanquish him — Harry — and using him to store a fragment of his soul? But what if Voldemort failed? It wouldn't be the first time he had forgotten the limits of magic — and there are limits.
The problem with Horcruxes -- other than the fact that they're evil -- is that there are no reference books for them. Even Hermione, expert researcher though she is, can't find anything other than a one-line mention in an old text called "Magick Most Evil" -- not much help there. So there remains the possibility that Lord Voldemort didn't know as much about Horcruxes as he thought -- and that the murders of Harry's parents were sufficient to split his soul, and the resulting fragment went into the vessel he was pointing his wand at: Harry. Voldemort simply wasn't capable of murdering someone and creating a Horcrux at the same time -- it was too destabilizing. It may be what blew up the Potters' house -- that, or the rebounding counter-curse. This could be the flaw in Voldemort's plan. He may have created enough Horcruxes to allow him to split his soul seven times — as Dumbledore guesses — but he doesn't fully understand them. He can't sense when they're destroyed (the diary, the ring), so perhaps he can't sense when they're created either. He's only aware of the ones he knows he created — he has no innate sense of where they are or how they're being used. "Where is Nagini?" Voldemort asks Wormtail — at a point in the story in which the snake Nagini has already been made into a Horcrux, according to Dumbledore's shrewd idea ("and Dumbledore's shrewd ideas usually turn out to be accurate," Professor Lupin reminds us).
So it would make sense that the only time Harry feels no twinge from his scar in Lord Voldemort's presence is when he's facing Tom Riddle — the young Voldemort — as he's embodied through the Horcrux diary. If Voldemort can't sense his Horcruxes, then the Horcruxes can't sense each other, either. "Time and space matter in magic," Professor Snape tells Harry. "Eye contact is essential" for Legilimency (mind-reading). So Harry and Voldemort can't be reading each other's minds — they're separated by too great a distance. And it can't be possession, either, as Ginny Weasley reminds Harry, since she had experienced possession in "Chamber of Secrets" — and the after-effect of possession is not knowing where you've been or what you've been doing. But Harry knows.
He knows where he's been during the visions. He knows what Voldemort is feeling. He knows how to speak Parseltongue (snake language). He knows "by instinct" to grab Quirrell's face before the professor can use a deadly curse, and he burns him with the remnants of his mother's love. "Without thinking," he seizes the Basilisk's fang and plunges it into the diary. He doesn't "understand why he was doing it, didn't know what it might achieve," but he forces the bead of light right back into Voldemort's wand when they try to duel, causing the Priori Incantem (the reverse spell effect). And there's also something in Harry's mind that helps him fight the Imperius curse — something stronger than himself.
The sorting hat sensed something in Harry's mind — that's why it considered putting him in Slytherin his first year (despite the fact he wasn't a pureblood). Harry's wand sensed something too — remember, the wand chooses the wizard, and Harry's wand and Voldemort's wand are brothers and can't be used against each other. As the prophecy said, Harry is marked as Voldemort's equal, all right — but not by something that can be explained by what we know so far about the wizarding world.
If it were just an Avada Kedavera curse that somehow rebounded because of Harry's mother's love sacrifice, why would the house be destroyed? Why would Voldemort leave no body? And why would Harry even have a scar in the first place? Some other spell must have gone violently wrong, because the Avada Kedavera curse famously leaves no mark, either on the victim or on the scene. "That is no ordinary scar," Dumbledore tells Harry. And why would it hurt Harry when Voldemort is nearby, even in disguise, except for when he's in the form of a teenage Tom Riddle? "I've never heard of a curse scar acting as an alarm," the Minister of Magic says. And when it's not a feeling of proximity but emotion, Harry also instinctively knows why. "Last time, he was pleased, he thought something good was going to happen." "He wants something done, and it's not happening fast enough." And Harry knows what else is happening. He hears laughter. He knows Avery is being punished, even though he doesn't see it. Could the pain Harry feels be the attraction of Voldemort's split soul to its former host? Note the wording — the pain is usually described as "bursting" or "splitting open." As if something needs to get out? Perhaps that's why the Dementors are especially attracted to Harry — they sense more than one soul for the taking.
Dumbledore has always understood more about this than he ever let on, telling Harry, "I guessed ... when I saw the scar ... what it might mean." What he didn't tell him was the full extent of what it might mean, calling it only a "connection." But there's a shadow of Voldemort in Harry's eyes. A shadow of his soul. A shadow he may not have meant to put there, and may not have known was there — which is why he still kept trying to kill the boy — who was ironically helping keep him alive. It's not a smart idea to leave your soul in something that can think and move for itself. It's also not a smart idea to take stock in partially overheard prophecies. For if Voldemort were to succeed in killing Harry, he would also be killing a part of himself. "Neither can live while the other survives."
Dumbledore wanted Harry to realize all this on his own — through his carefully worded lessons — because how do you tell a boy that he's got to sacrifice himself to save the world? It has to be his realization, his choice — which is, after all, the overriding theme of the series. As Dumbledore says, it is our choices that show what we truly are. And Harry, well, he has a doozy of a choice ahead of him now.