I suspect that if Google put it's collective brain power on it they could do what we've done and not rank comment links. One of the 'easy' things to do is run your ranker on the crawl where you disregard any reader contributed content. That does wonders. What is less doable is when the blogger is in on it (as they were with the recent scandal) and the links are in the page.

But here is the thing, Google has a content evaluator as part of their Adsense for Content product and they should be able to compute the equivalent of the 'hamming distance' between the linked pages and the content on the current page and construct a decent rank. Certainly that is what I would do if I had the resources to crawl as much of the web as they do.