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In Harry Potter and the Sorcerer's Stone, the Mirror of Erised is said to be the ultimate protection against the impure of heart who desire to obtain the Stone only for their own desires.

However, the Mirror itself is not stored in Dumbledore's forbidden third-floor corridor until practically the end of the book. Instead, it appears to be collecting dust in an unused classroom at least five floors above the kitchens, until Harry finds it and Dumbledore moves it, following their discussion.

It's assumed by the characters that the Stone is hidden underneath Fluffy in the third-floor corridor since it was moved to Hogwarts from the Gringotts vault, but given that the Mirror seems to be the only true protection against a powerful Dark Wizard, wouldn't it make more sense to hide the Stone in the Mirror from the very beginning?

Do we know anything else about whether the Sorcerer's Stone was stored in the Mirror of Erised before it was moved into the forbidden third-floor corridor?

In Chapter Eleven of Philosopher's Stone we have the following passage:

"I'm tellin' yeh, yer wrong!" said Hagrid hotly. "I don' know why Harry's broom acted like that, but Snape wouldn' try an' kill a student! Now, listen to me, all three of yeh – yer meddlin' in things that don' concern yeh. It's dangerous. You forget that dog, an' you forget what it's guardin', that's between Professor Dumbledore an' Nicolas Flamel –"

Thus – at least as far as Hagrid knew – the Stone was at that point already guarded by Fluffy. Harry didn't discover the Mirror of Erised until the next chapter, which would mean that the Stone was not protected by the Mirror at that point.

However, there is also an implication that the Stone might have already been in the Mirror at that point. In Chapter Twelve, Dumbledore explained to Harry how the Mirror works, and then explicitly mentioned that this would prepare Harry if he ever came across it again:

"The Mirror will be moved to a new home tomorrow, Harry, and I ask you not to go looking for it again. If you ever do run across it, you will now be prepared.

In Chapter Seventeen Harry thinks that Dumbledore did this purposely in order to give Harry a chance:

"No, it isn't," said Harry thoughtfully. "He's a funny man, Dumbledore. I think he sort of wanted to give me a chance. I think he knows more or less everything that goes on here, you know. I reckon he had a pretty good idea we were going to try, and instead of stopping us, he just taught us enough to help. I don't think it was an accident he let me find out how the mirror worked. It's almost like he thought I had the right to face Voldemort if I could...."

In order for this to make sense the Stone would have had to already been in the Mirror back when Harry discovered the Mirror, or at the very least Dumbledore would have had to already have thought to store the Stone there. And if he had already thought of the idea, there doesn't seem to be much reason why he wouldn't have executed it immediately. Of course, though, there wouldn't be much point in having Fluffy and all the subsequent traps if the Stone wasn't even hidden there, unless the whole third-floor-corridor thing was just a decoy.

The plot of Harry Potter and the Philosopher's Stone only really makes sense if you assume that Dumbledore was not actually worried about the prospect if Quirrel retrieving the Stone and has actually set up the whole plot as a mechanism to train Harry.

Dumbledore appears to be aware from a relatively early date that Quirrel is a traitor. For example, he seems to know all about the episode with Quirrel and Snape dueling for control of Harry's broom. Moreover, the headmaster arranges for Harry to be familiar with the Mirror of Erised, and he also sends Harry to detention in the Forbidden Forest, when there is a supposedly unknown killer loose in the Forest, hunting unicorns. The events in the Forbidden Forest provide Harry with important experience and give him a greater notion of what is at stake. However, sending kids out into the path of a murderous monster would have been incredibly poor judgement, if Dumbledore had not been in complete command of the situation.

So it seems that Dumbledore was actually pretty confident that Quirrel was not going to get through the barriers protecting the Philosopher's Stone until Dumbledore decided Harry was ready to face down Voldemort. (Note also that the protections placed around the Stone just so happen to be ones that talented first years should be able to get past.) The Mirror of Erised was needed as part of Dumbledore's plan for how that encounter would play out, but until that final confrontation, it was not necessary. In all likelihood, the Stone was waiting in the same room where the Mirror was ultimately located, just without the Mirror. Only when the final faceoff between Harry and Quirrel/Voldemort was about to happen did Dumbledore need to place the Stone inside the Mirror.