http://www.tizish.com – In this video I cover the URL issues of dynamic versus pretty permalinks (keyword based url structure), www versus non-www based urls, and lastly the ‘Trailing Slash’. All of these items can hurt your SEO efforts big time if you link to your own content or others link to your content with different versions of each. Watch the video to make sense of it. But basically if your website is accessable through different url’s that show the same pages then you can have duplicate content issues and this will hurt the flow of “link juice” through your site, and of course this will hurt your seo efforts.
Here is the article on Trailing Slashes I reference: http://googlewebmastercentral.blogspot.com/2010/04/to-slash-or-not-to-slash.html
Here is the article on the www versus non www issue I reference: http://www.mattcutts.com/blog/seo-advice-url-canonicalization/