You’ve worked hard to prevent any duplicate content on your website. No copy-pasted content, no two URLs returning the exact same webpage due to incorrect usage of mod_rewrite, and so on. But then, a couple days after the big launch, Google starts getting cluttered with dozens of references to your website, which harms your rankings. All due to an incorrect and insecure alternation of the PHP server by enabling the use of ?PHPSESSID in the URL.
Full article: http://www.seomoz.org/ugc/removing-phpsessid-from-an-url