You can use your .htaccess file to ensure that all pages load the way you want, with or without the "www." so all bookmarkers and spiders will see the address you want to favor.
Here's an example of what goes in the .htaccess file if you want to use the
www...
RewriteEngine On
RewriteCond %{HTTP_HOST} ^yourdomain\.com
RewriteRule (.*) http://www.yourdomain.com/$1 [R=301,L]
That's all. And just edit as necessary if you want things the other way.
HTH