Google has put out a new group of videos about various webspam topics. Three of these are specifically about unnatural links. Here's one on unnatural links from your site, and here's one on unnatural links to your site.
While both of these videos featured Matt Cutts with other Googlers, this one is just Cutts himself talking about unnatural links and their impact.
"Over time, we've gotten more granular, and our approaches have become more sophisticated, and so as a result, if you think perhaps that your site overall is good, but there might be some bad links (it's not all bad links, but a portion bad links), then we might be taking targeted action on just those links. And that can be bad links or links that you might not think of as typically bad."
He goes on to talk about various examples. If you've got about ten minutes to spare, you'll probably want to give it a watch.