After my analogy between LEGOs, Play-Doh and CSS (see "A Bettery Way for Organizing CSS"), I'll go out on a further limb and compare the best practices for URL canonicalization with what the Catholic Church has been doing since the Second Vatican Council.

So like church canon, your URLs need to point to definitive and unique sources if you don't want to get caught up in a duplicate content filter on the major search engines.

I explain the analogy in further detail on the flexible philosophy with my URL Canonicalization, the Catholic Church and Your Web Site entry, but on Demystifying the Web I'll save you the reading and provide you with the SEO best practices:

If your site is hosted on an Apache Web server, you want to set up .htaccess files to make sure all URL requests point to either a www or non-www version of your domain. We also want to fix the problem of trailing slashes and default document redirects. Both of these things will shave off your typical duplicate content problems on new domain installations.

Follow the steps in my URL canonicalization entry after you create an ASCII encoded file called .htaccess in your root directory.

Oh yeah, this does not apply to those who are hosted on IIS servers unless you have software that imitates Apahce's mod_rewrite functions.