Just an update here... I mentioned before that I had had a link out to a site that looked perfectly nice, didn't have dozens of spammy links in the footer or Adsense blended in or anything, but I realized in hindsight that site was targeting a keyphrase very heavily.
Well, in second hindsight, I realized I linked to the site with that keyphrase in my link, so Google probably thought I sold them that link. Would that explain the Penguin slap? Paid links are a definite no-no, but not necessarily an SEO thing (people who know nothing about SEO can be asked to sell links, and do so for the money, not because they understand how pagerank works or that Google even has an opinion on the topic). But I call this one a "maybe."
I removed the link in November, and I've begun to get some VERY long tail traffic to that page, which wasn't happening before.
Today I filed for reinclusion (I've done it before on this site, so it can't hurt to do it again) and explained precisely why I thought they had believed the site was violating guidelines, and what I had done to make it comply.