At Google Webmaster central bloged appeared a news about improving your website duplicate content awerness for Google search engine. This is very good, we should have much more tags helping search engines index and most important understand the nature of the page and content.
Adding something like a
<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />
to a page you specify a preferred url for Google to index for that page content. This is needed because entry points for same content can be through different URLs but you might want to make one favorable by Google indexing.