tedster - 3:48 am on Oct 1, 2012 (gmt 0)
Right now, I'm not really considering backlinks as a factor. There was nothing in the Tweets to indicate it was anything off-page. And besides, Google has already hammered low quality backlinks quite a bit this year.
So I'm leaning strongly to on-page and on-site factors. If I had to fix a site that was hit, that's certainly where I would start looking, at any rate.