I'm prone to think that pg's algorithm is, to some extent, content based. I have no real evidence to support this, just the evangelism in "A Plan for Spam."
Right. That's what I meant by "spammers correlate with each other". I'd be very surprised if content-based filtering were useful for catching voting rings. You can use graph-theoretic metrics to catch them, but unlike spammers, identifying one doesn't obviously help you identify others.
