Setting the preferred domain is refering to how the domain is displayed in the SERP. It never fixed the canonical URL issue which many webmasters mistakeningly thought / think that it did / does.
Canonical URLs is not much of a duplicate content issue, so much as it dilutes the PageRank of your site with links that point to
http://www and www.
Diluting the
PR of your site would as most know hurt your keyword rankings at times.
Best thing to do so that all engines affix the proper scoring to your website, is to fix things via mod rewrite for Apache Unix Linux boxes, or Isappi rewrite for windoze.
Below is the code for modification of apache server via the htaccess file
RewriteEngine on
RewriteCond %{HTTP_HOST} ^yourwebsite\.com [NC]
RewriteRule ^(.*)$ http://www.yourwebsite.com/$1 [L,R=301]
I have no clue how to fix it on windoze servers.....
Hope this helps
The original Clint ;->