On the Google Webmaster Central Blog today Amit Singhal, Google Fellow, and Matt Cutts Principal Engineer, announced that an algorithm change is being rolled out that will affect 11.8% of searches. That’s massive. The reason for the algorithm change, as always is to improve the quality of the search results, specifically to down grade low quality sites. These are sites that are basically copying content from other sites or “low value add” whatever that means.
The reasons for this tweak seem pretty obvious: to cut down on the influence that dummy sites and "link farms" have on search-result positioning. In most cases - especially for legit sites that don't engage in any attempts to fool Google - this change will have no effect.
But if you're trying to game the system...
More info at Stewartmedia.