A while ago I suggested a possible Google-side solution. The basic idea is that comment spammers are easily identifiable by a simple pattern, namely that they create an unhuman amount of comments in a very short time period. It shouldn’t be difficult for a search engine to parse out the dates associated with comment posts and limit the pagerank contributions at a treshold.
That way the basic propagation of pagerank would still work for actual comments, as fluffy suggests above.
If Google actually does something like that, they should be public about it in order to actually decrease comment spam (as opposed to “just” increase the relevancy of their results by that means)