There's a theory going around about how it was Amelia Bones who killed Narcissa Malfoy, based on nothing more than the stray thought 'Someone would burn for this.' What she said to Dumbledore during Hermione's trial ('You know the answer you must give, Albus. It will not change for agonizing over it.') seems to be taken as further evidence.

sigh

Of course it was Voldemort who did it!

I think what most of you fail to realize is, the whole thing happened after Voldemort heard the prophecy from Snape. Dumbledore predictably ignored the blackmail, and would not have killed anyone like that in retaliation (he would never crack, no matter what), but was willing to falsely claim he did it - one lie to an enemy is a small price to ensure no further loss of life to blackmail attempts. This resulting in Malfoy hating Dumbledore was also predictable. His hate kept the conflict alive after Voldemort's planned disappearance. This kept the lines of battle predictable, set the stage for Voldy's planned return.

I don't recall seeing anyone state this, and I really hope someone did - it would suck to be the only person who could realize something simple like this.