Many websites work with either “www.mysite.com” or “mysite.com”. Google doesn’t like that since it looks like two sites–sometimes they might penalize you for it.
From their guidelines page:
Don’t create multiple pages, subdomains, or domains with substantially duplicate content.
Here’s an excerpt from my .htaccess file that rewrites the URLs without the “www.” to include it:
# rewrite 185vfx.com to www.185vfx.com
RewriteCond %{HTTP_HOST} !^www\..*
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} ^([^.]*)\.(com|co\.uk)
RewriteRule ^.*$ http://www.%1.%2%{REQUEST_URI} [R=permanent,L]
I grabbed this code from a discussion board somewhere a while ago but I don’t have the link handy to give credit where credit is due.