Google announced last Friday that they are gonna allow you to block useless sites. The gist of this feature is that when a link farm shows up in your search results, a link allowing you to block that site from future results is provided. Pretty cool. This will be a great help on some of those rare searches that result in nothing but link spam.
However, there is a danger if Google wants to use this feature to improve search results. Remember the first immutable law of the Internet. The first thing spammers will do is use this feature to block any good content. It is inevitable and predictable. Hundreds of spam servers requesting that any good content be blocked.
But I expect Google will figure a counter strategy in short order.