An original editorial by Daniel Schwarz
As you know, Dumbledore has a brief glint of triumph in his eyes when Harry tells him that Voldemort was able to touch him, as a result of being resurrected with Harry's blood. Harry is now vulnerable to his mortal enemy. However, a moment later, Dumbledore looks old and weary, resigned to saying "Voldemort has overcome that particular barrier." What is going on here?
One theory is that Dumbledore is truly in league with You-Know-Who, and thus pleased that Harry can now be destroyed by him. Yet, this really doesn't make much sense. If Dumbledore were evil, why would he have fought against Voldemort for all those years? The two of them together could easily spread horror throughout the wizarding world.
While it is easy to dismiss such a fleeting instance, I believe there is significance in what happened. Instead of being triumphant upon learning about Harry's mortality, Dumbledore may be pleased because this news means the same for Voldemort. Having used Harry's blood to give him a new body, Voldemort has made himself vulnerable to our hero. Voldemort spent the years after leaving Hogwarts transforming himself with dark magic so that he could never be killed. Now that he shares Harry's blood, I think Harry can destroy him for good. Dumbledore is aware of this, and that is why he is pleased.
Further evidence for this theory can be found in the first and third books of the series. At the end of the third book, Harry asks Dumbledore why Voldemort wanted to kill him in the first place. Dumbledore says that Harry will find out when he is ready. Dumbledore knows the answer, but is unwilling to tell Harry. This is also why Dumbledore tries to hide his triumph in book four... he can't let Harry know about his own destiny. I use the word destiny with the idea that the secret for why Voldemort attacked the Potters in the first place is found in the first prophecy made by Professor Trelawny mentioned in book three.
Whether the prophecy mentioned Harry being an heir to Gryffindor or not is debatable, but it is reasonably logical that the prediction included Harry's fate for destroying Voldemort. Had Voldemort learned of this prediction, he would naturally want to kill Harry before it could be fulfilled.
Going back to book four, we find that Dumbledore is seemingly unaware that Moody has been replaced by an imposter. This, however, does not go along with the Dumbledore of other books, who can always discern the true from the untrustworthy and always has solutions to every problem. Perhaps he allowed Harry to be taken by "Moody" just as he let Harry go after Quirrell in book one.
Fate and destiny are bound to become central themes in the coming plot. We already see them in the first novel as the centaurs discern the meaning of the unusually red Mars. Could Harry's fate be written in the stars... and is Dumbledore just seeing that destiny is fulfilled? What else is destined to be in the world of magic?
Before concluding, I would like to mention one more theory about the books. In book three we discover that Dumbledore had a spy inform him that Voldemort was after the Potters. In book four, we learn that Snape was a spy for Dumbledore. Snape owed his life to James Potter, ever since James saved him from Lupin's transformation into a werewolf back in their Hogwarts days. I am convinced that Snape gave Dumbledore information about Voldemort's aggression against the Potter's to pay his debt.
Perhaps, this was the only reason Snape spied on Voldemort in the first place. Snape seems very evil, and was in the Slytherin house. He was a Death-Eater, and it seems unlikely that anyone could successfully spy on Voldemort since Voldemort has the ability to detect lies. In that case, contrary to what Dumbledore believes, Snape may still be loyal to Voldemort. After all, the only nice thing he ever tried to do was save Harry, and that was only to repay Harry's father.