Search engines don’t assume that http://www.domain.com and http://domain.com are the same site; they don’t need to be, and can be owned by different parties and have totally different content.

How do you make sure that the search engines are seeing the URL you intended?

I’ve been setting up 301 redirects in my clients’ .htaccess files for a while now, to make sure that when someone searches on ‘domainname.com’ they get pointed to ‘www.domainname.com.’ I just think it looks better and more professional to keep it consistent.

Here’s an informative article and discussion by Matt Cutts on canonicalization.