There seems to be some confusion about the prophecy as explicated at the end of Book 5. I thought I would give you my take as to what it means and doesnt mean. Take it or leave it.

Just for the record, let's set the text down:

The One with the power to vanquish the Dark Lord approaches...Born to those who have thrice defied him, born as the seventh month dies....and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not...and either must die at the hand of the other for neither can live while the other survives...The one with the power to vaquish the Dark Lord will be born as the seventh month dies..."

Dumbledore tells Harry that this prophecy could have applied to two people: Harry and Neville. Upon hearing this, Harry is hopeful that perhaps this was all a big mistake. Call up Voldemort, let him know, he's got the wrong guy. No such luck, says Dumbledore:

"Then -- it might not be me?" said Harry.
"I am afraid," said Dumbledore..."that there is no doubt that it is you."
"But you said -- Neville was born at the end of July too -- and his mum and dad --"
"You are forgetting the next part of the prophecy, the final identifying feature of the boy who could vanquish Voldemort...Voldemort himself would 'mark him as his equal.' And so he did, Harry. He chose you , not Neville. He gave you the scar that has proved both blessing and curse.
"But he might have chosen wrong!" said Harry. "He might have marked the wrong person!"
"He chose the boy he thought most liekly to be a danger to him," said Dumbledore..."and in marking you with that scar, he did not kill you, as he intended, but gave you powers and a future, which have fitted you to escape him not once, but four times so far..."

Dumbledore's answer to Harry is that there was no wrong choice for Voldemort to make. Whomever he saw as the greatest threat, whoever he marked, would be his enemy, and eventually have to face him. This fits with JK Rowling's overall theme of personal choice: we are not defined by our abilities, but by our choices. Harry became the Prophecy Boy, not because he is the great-grandson of so-and-so, or the mixture of this house or that house's bloodlines. Harry became the Prophecy Boy because Voldemort chose him. Had Voldemort chosen Neville, it would be his burden to carry.

A little insight in to this, I think, will show us where things are going to go with Harry and Voldemort. Voldemort's essence (indeed, even his name, the French Vol-de-Mort) represent a Flight from Death. Voldemort is terrified of one thing only: death. He does not wish to die, to fade from this world. Because of this, he can not love. When we love, we invest a part of our selves in another person, but in doing so, we open our selves up to pain and loss. Because when a loved one dies, when they leave this world, they take a part of us with them, and we can never get that part back. It is a bitter bargain that we make. Voldemort, fearing Death in any form, refuses to love, refuses to invest part of himself in others. When he hears of one who will vanquish him, he sets out to destroy that person, and all connected to him.

And here is the trick. Voldemort goes to kill Harry and his parents. Harry, who is saved by the love of his mother, has a preternatal connection to both Death and Love. Part of Harry is gone from the beginning, his parents having sacrificied themselves to save him. Harry knows of Death, and is not afraid of it. In the Death Room at the end of Book 5, Harry is drawn to Death, to the voices behind the veil, unlike the others (apart from Luna). Harry's connection to Death allows him to love without fear. He has experienced Death, the pain that comes from losing someone you have invested part of your soul in.

I hold it true, whate'er befall;
I feel it, when I sorrow most;
'Tis better to have loved and lost
Than never to have loved at all.

In Memoriam, Alfred Lord Tennyson

This knowledge of Death gives Harry the ability to Love. His soul is already incomplete, his parents having died before he knew them. But this gives Harry the ability to love, to invest in others without fear of Death taking it away.

And who gave this to Harry? Voldemort did. By killing his parents, Voldemort created the exact type of person who will have the power to defeat him, the person who Loves, and does not fear Death. Dumbledore says as much to Harry, intimating to him that his one strength over Voldemort comes from his ability to love. It is with this Love that Harry will overcome Voldemort, unafraid to die to protect that which he cares about.

So Neville is not the guy; his life experience does not give him the proper knowledge to ultimately defeat Voldemort (that is not saying he isnt a great guy...). The marking of Harry by Voldemort created in him the unique consciousness of Love and Death that is required to take on he who flees from Death at all costs. He, and only he, is the Prophecy Boy.