EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
Anyways, if the obvious answer is incorrect, we ought to figure out which Hogwarts staff-member has been going around casting memory charms. Now Dumbledore did specifically say "professor" and I doubt that he misspoke, so we can discount Hagrid, Pomffery, etc. So:
Now, there is one possibility I don't think anyone has brought up yet. H&C could simply be the unwilling pawn anyone capable of using the Imperius, This explains how he/she can have quite brilliant long-term plans, given to them by their puppet-master, but make a few simple mistakes when on their own (Like taking so long to crack Hermione).
Now, we know that Hermione recognized and was very shocked by whomever she saw beneath H&C's disguise, which suggests it was someone she actually knew rather than a professor she occasionally caught sight of at dinner. First years take Astronomy, Charms, Battle Magic, Herbology, History of Magic, Potions, and Transfiguration. We can safely eliminate Quirrell, Snape, Binns and McGonagall from being imperiused, leaving Flitwick, Sprout, and Sinistra.
I would not take that as a reason to think that person is unknown to Eliezer - he's well-versed in both HP fanfiction and wiki.