A patent granted to Google today describes a way that the search engine might use to identify two different methods of spamming pages, and take action against artificially inflated importance (or PageRanks) for pages.

This method of identifying link spam involves looking at a sampling of links to a page to see if the search engine can identify certain characteristics that appear in a couple of different types of manipulative linking.

source: http://www.seobythesea.com/?p=1248