I have always though that canonical urls was a must fix for every site, because it was logic consecuence of the famous  'Duplicated Content' issue.  Days ago, Matt Cutts (Google Enginer) has confirmed the issue and even presented a,... lets say, tag solution (a line code we should put in between the head tags), this tag could be solving the canonical urls issue.

We should say that:

www.example.com

example.com

www.example.com/index.php

example.com/index.php

www.example.com/home.html

and so on... Are Not The Same site for the google crawler's eyes, thus, we are not playing well or 'legally' about the Duplicated Content Rules :) so you can get hit/penalized by google search engine (your page rank can drop dramatically)

Solutions:

As per page solution:

<link rel="canonical" href="http://example.com/page.html"/>

As per site entrance solution:

In your .htaccess file add this

RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

With this addition your site will load ALWAYS as yoursite.com whatever your visitors use www. or not .

As per Joomla users:

Please refer to this website's article 

" That's ok, but I dont care about all this! " - Yes no problem you can continue using your site without fixing this canonical issue but be sure that your site is loosing positions in google dramatically, because simply your site looks like is having/using duplicate content. That simple. Are you still don't care about it?