When I first read the book, I just assumed that it must have been needed for some operation or another that Dumbledore had been cooking up at the time. Maybe even a diversion plan to lure Voldemort away from Godric's Hollow, as swythyv explains in her very convincing essay. But something still doesn't seem to fit the picture completely, if we decide to take that view. For one thing, why would Dumbledore, already dead and now defenseless against Harry's full knowledge of his past deeds (or so we think), imploring him for forgiveness and absolution from beyond his grave, still choose to tell him a blatant lie?
I mean, Dumbledore may be a pretty horrible man personality-wise, but he still embodies the Side of the Good with all his being, in all of his outward behaviors. He never -- or almost never -- straightforwardly lies, at least without good reason (judged by his own warped sense of what constitutes the Greater Good, of course). He omits, he prevaricates, he deliberately misleads, but he doesn't usually outright lie... Unless, of course, when he deems it absolutely necessary. He deemed it necessary to lie to Harry in PS/SS that Snape's motivation for protecting Harry's life was one that was hateful and vengeful, as opposed to saying he had no idea or hinting that the motivation might be related to any emotion more akin to Sirius' reason for taking a liking to Harry, for whatever deluded reason he had (such as, oh, the unmistakable fact that he didn't want these two major pawns of his bonding with each other and later deciding that Dumbledore's plan to sacrifice Harry was one that was unpalatable enough for them to not want to follow through with). He deemed it necessary to lie to Harry in HBP when he told him that "he hoped" those pensieve lessons would help him to survive. That was a straight lie, not even prevarication (which, when he told Harry about Slughorn's new recruitment in slippery enough words that Harry was led to believe he was helping him get a new DADA teacher, I think it was), because at that point he already knew that his final plan for Harry was suicide (or attempted suicide, depending on how much of a chance he thought Harry had of surviving), and Harry's knowledge of Voldemort's parentage etc. clearly had nothing to do with whether he would survive Voldemort's murder curse. And Dumbledore knew this. But like he had a good reason for not letting Snape tell Harry about the suicide mission until he'd destroyed most of the Horcruxes ("otherwise how could he have the strength to do what must be done?"), so did he have a reason for telling Harry that the lessons he was getting from Dumbledore in HBP -- and the subsequent order that they led to -- were going to help him defeat his enemy and come out of the war alive. So it's not that Dumbledore never ever lies, but when he does, he tends to have a very strong, strategic reason for it. Then the question is, what motivation could he possibly still have, to keep on lying from beyond his grave?
Because it's either that he did lie, about whatever sensible war-effort type thing he had really been planning with that Cloak, or that he wasn't lying and he really had chosen to take away a strong magical protection item -- the one thing that might have given Lily a split second advantage, had she had it on hand (would you let go of such an item for even a second if you were the mother of a baby targeted for murder? [*]), so that she could leave Harry's side to go get her wand and fight rather than be left with no option but suicide -- for nothing more than some whimsical curiosity of a completely personal nature.
Then I was re-reading Snape's pensieve memories, and one scene struck me for how different Dumbledore's words there are from the way he phrased things to McGonagall on the Dursleys' doorstep, sometime within the same twenty-four-hour time frame -- which we had read way back at the beginning of PS/SS:
"It's -- it's true?" faltered Professor McGonagall. "After all he's done... all the people he's killed... he couldn't kill a little boy? It's just astounding... of all the things to stop him... but how in the name of heaven did Harry survive?"
"We can only guess," said Dumbledore. "We may never know."
[Philosopher's Stone, Ch.1]
The thing is, we were to find out later that Dumbledore says this kind of a thing a lot. He's always quite precise in choosing to say either "I'm guessing" or "I know this," as the speculative history lessons in HBP has shown to us ad nauseum. Not that we knew such a character trait of his back in PS/SS...
So I was kind of taken aback when I went back to read that again post-DH, and then came back to Snape's pensieve scene to make sure I hadn't misread anything:
"If you loved Lily Evans, if you truly loved her, then your way forward is clear."
Snape seemed to peer through a haze of pain, and Dumbledore's words appeared to take a long time to reach him.
"What--what do you mean?"
"You know how and why she died. Make sure it was not in vain. Help me to protect Lily's son."
[Deathly Hallows, Ch.33]
Now hang on. He knows how and why she died? How and why she died? How on earth could Snape know that? Who told him? Who could?
Knowing Harry was charmed by Lily's protection by looking at him, or sensing the aura around him or whatever, now that's one thing. I'm sure that kind of deductive thinking could have been enough for Dumbledore to "guess" that Harry was protected by Lily's love protection charm, and correctly plan his next move -- the end result of that planning being his arrival at the Dursley's where he talks with McGonagall and plants the baby on its relatives' doorstep. But knowing something, anything, about how and why she died is quite another thing. It sounds like Dumbledore is talking about the way Voldemort told Lily that she didn't have to die, the way she begged and cried for him to take her instead of Harry, the way she wouldn't step aside no matter how many times Voldemort insisted, the way she had no wand at hand to fight him with and had no choice but to hope the sacrifice of her life would be enough to protect her baby (which incidentally hadn't ever been enough in the known history of the Wizarding World before she managed to pull it off). In other words, those facts we're by now quite familiar with, having seen them in Harry's and Voldemort's memories. Yeah, that was a heart-wrenching manner and circumstance of death. If anything can motivate the heartbroken, despairing and near-suicidal ex-Death Eater to keep on living and working for the Good Cause, it's got to be that. But the thing is... Is Dumbledore just lying to Snape here, feeding him some empty speculative tale (one that might be close but can't possibly be exactly the same as the actual facts) in a language that only pretends to be authoritative about the truthfulness of the whole matter? Because how could anyone know exactly how she died, except Voldemort himself and the Potter family? Of whom, I might add, every single one but the one-year-old, not-yet-comprehending-of-human-language Harry were dead or vaporishly undead at that point in time?
Except, of course, if there had been a spy planted in Godric's Hollow, one who could see into the Potters' house (i.e. was told of the location by the Secret Keeper) and could observe the events going down without being witnessed by Harry or Voldemort -- both of whose memories of the event never hold a trace of any third party. An invisible observer. And an extremely inactive one.
Could Dumbledore have been standing right there when it all happened, under the Invisibility Cloak, holding his Elder Wand raised at the ready in case something went wrong -- which it didn't? Or could he have planted somebody else with the Potters, unbeknownst to the couple themselves, watching everything, making sure that Harry got attacked by Voldemort and subsequently survived, not lifting a finger to change the course of events as long as things were going well? Am I reading too much into just two little words "know" and "guess," maybe, possibly?
A part of me kind of thinks I am. God, I hope I am. But lately I'm just finding Dumbledore's predisponency to use different words for each different audience really very frightening indeed...
[*] But then again, would you leave your wand somewhere out of reach, ever, if you knew your child was targeted specifically? Both you and your husband at the same time? Even when you slept, wouldn't you put the protection stick under your pillow, just in case, no matter that the house was already protected, if the threat was on your baby? Would you be laughing indulgently about your husband being frustrated at having to stay at home with nothing better to do than protect his wife and child, wishing he had the disguise item he could use to go away and have more interesting adventures? Which makes me wonder if Dumbledore ever even let the Potters know that it was Harry that Voldemort was specifically after. I mean Harry was born on the very last day of July. Even Muggles have some technology that could have controlled Lily's pregnancy and/or held off her labor/delivery so that Harry would get born sometime past midnight -- or hey, pretend he did, by begging the doctor to lie in the birth certificate.