You can set your .htaccess file to set a permanent redirection for either with or without the www, your choice. I recommend every webmaster does this for PR reasons.
Type in yahoo.com, and it will add the
www. This is how I set up my sites. It doesn't matter if someone links to my site with or without the www, the .htaccess settings redirect to
www.sitename.com and Google counts it as such.