I had a similar problem, and this .htaccess works for me
RewriteEngine On
#This bit rewrites your host name to include www
RewriteCond %{HTTP_HOST} !^www\.example\.com [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,NC,L]
#This bit does the codeigniter magic
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php/$1 [L]