Thanks; I get to work with a wonderful/smart group of people at Google--they're doing the really hard work. Both webspam and search quality folks are crunching on linkspam and content farms, so I hope we'll have more to talk when some of the things in the pipeline make into production.
We've talked a lot about why buying/selling links that pass PageRank violates our guidelines. The best starting point is probably this blog post: http://googlewebmastercentral.blogspot.com/2007/12/informati...