g1smd - 10:25 pm on Apr 12, 2011 (gmt 0)

Setting options in WMT signals only to Google which URL version you prefer.

It does not signal anything to other searchengines, or to users, and it does not stop further links being built to the "wrong" URL.

Setting the .htaccess rules signals to all users and all searchengines which one is "right".

A more robust ruleset...

RewriteCond %{HTTP_HOST} !^(www\.example\.com)?$
RewriteRule (.*) http://www.example.com/$1 [R=301,L]

Using www URLs is usually preferred, but remove www\. from BOTH parts if you need to redirect to non-www.