Yesterday, word came from Google, Yahoo, and Microsoft on a cooperative effort, and a brand new page tag, to help eliminate indexing of duplicate content.
If you've ever run large websites, particularly those using content management systems, control of these pages in many cases has been somewhat difficult. On the other hand, Search Engines have warned repeatedly about duplicate content and rankings, so the new 'canonical' page tag is a significant development and should be noted by ALL in the SEO world.
Rather than duplicating content myself, an excellent piece by Vanessa Fox including links to the posts from Google, Microsoft and Yahoo! can be found at Search Engine Land here.
This is must reading. Period.
Great weekend all!
Charlie
Update 1 - Google's Matt Cutts just posted an in-depth explanation of the new canonical tag here.