So, what's the importance of Roger Bacon's diary? Canon & conservation of detail both suggest it's something, possibly a horcrux or possibly some other tool of Quirrelmort. This Voldemort is too smart to horcrux his own diary, but this diary would be an awfully convenient trojan horse for him to have (extremely durable, treasured by Harry).
It doesn't seem to produce any sense of Doom, though, which seems to count against the horcrux hypothesis.
Could Quirrell be using it to spy on Harry, to get his curiously accurate priors? Does Harry keep the diary in the pouch which he carries around everywhere, and which he brought to Azkaban, and which the Quirrell cast a spell on and entered in snake mode? If so, it could be a part of Quirrell's current plot. (Or, Quirrell could've done something else with Harry's pouch, which doesn't involve the diary.)
If it is a "very recently" created horcrux, one thing it means is that the maker is healthy enough to make horcruxes. Taken with Ch54, killing-curse ricochets might not be as damaging as one expects.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.