Google, Microsoft, and Yahoo have added support for the new Canonical Link element to help identify a link to content on your site where all traffic should land when more than one link exists to the same content. The Canonical Link will be used for the search engine index, and sitemap URLs will still be used when there is a tie between more than one URL link.
WordPress 2.9 has now integrated the rel=”canonical” link element, which will display between the “head” tags, read more