The new thread, discussion 13, is here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With three chapters recently the previous thread has very quickly reached 1000 comments. The latest chapter as of 25th March 2012 is Ch 80.
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, eleven.
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Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).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.
Other than the "external database" option, the only other sources of name information I can think of are:
I feel that picking someone's name from their own mind seems the most elegant and consistent. It doesn't handle babies (Before the parents choose a name, can a baby even be said to have one? Babies would have to be special-cased regardless), but it does allow arbitrary people to be mapped (multiple strangers being indistinguishable from each other seems like a serious flaw in a security system) and requires no external registry. On the one hand, it seems like interrogating the mind of every human is vastly more complicated than just looking up the name in a database, but to the kind of epistemology which would seem obvious to a 9th-century witch or wizard I can see it being "obvious".
(And to respond to your question about Pettigrew in the great-grandparent, I would assume that the map skips over animals entirely, which would probably include animagi. This would tend to lend a slight amount of weight to my "the map displays your name as you know it" theory, as if the names came from how everyone else around you knew you there would be no reason not to include pets.)
If my theory is true, it raises an additional interesting question: Is it possible to obliviate yourself selectively so that you lose all knowledge of your own name? (Possibly storing the memories in a pensieve first so you can recover them later) And if so, is the map the only piece of the Hogwarts security system which might be impeded by this?
A further idea: Professor Quirrel is shown to take a very loose approach to identity and names ("Identity does not mean, to such as us, what it means to other people.") Possibly Quirrelmort is the constant error, not because his name is wrong, but because he doesn't have a name attached to his marker at all.
I'm not saying this is true. But I hope it is because it would be awesome.