How to 301 redirect non-www to www url

[eluser]Mischievous[/eluser]

Code:

RewriteEngine on
RewriteCond $1 !^(index\.php|images|robots\.txt)
RewriteRule ^(.*)$ /index.php/$1 [L]

found @ http://ellislab.com/codeigniter/user-gui.../urls.html