Google, Yahoo, and Microsoft announced today a joint effort to help reduce duplicate content. The three major search engines came together to allow users to point out their preferred version of a url. As Matt Cutts explains in this video, this format offers users more control.
Duplicate content has been a challenging issue for a long time. Websites containing a lot of content such as a retail site, could end up with several urls for each page making it difficult for search engines to crawl.
Google gives an example on the Webmaster Central blog.
Watch this video of Matt Cutts at SMX:
For more information, each of the search engines have explanations and examples in their own announcements: Google, Yahoo, and Microsoft.