Google made a change to it’s search engine algorithm to drive search engine spam levels lower.  This primarily affects sites that copy others’ content and sites with low levels of original content.  The net effect is that searchers are more likely to see the sites that wrote the original content rather than a site that scraped or copied the original site’s content.

According to Matt Cutts (The head of Google’s Webspam team), this change affected about 2% of all queries.