This is useful for sites that store a cookie for each version of below.. this will redirect all non http://www. users to http://www.site.com, not an issue normally unless users are logging in to an account.
RewriteEngine On RewriteCond %{HTTP_HOST} !^http://www. RewriteRule ^(.*)$ http://http://www.%{HTTP_HOST}/$1 [R=301,L]