for my setup this is what i have for the default site, if they type in an IP address or if there is no site associated with the a domain name. i put it at the top of my _default.vhost file and i dont have a ServerName key in the section:
Code:
<VirtualHost _default_:80>
DocumentRoot /var/www/html/
ServerAdmin webmaster@example.com
ErrorLog /var/log/httpd/default-error_log
CustomLog /var/log/httpd/default-access_log common
</VirtualHost>