Getting "Domain Name Unavailable to Access" error

I have a client attempting to map a domain to my multisite network, and I'm getting a "Domain Name is Unavailable to Access" error on this.

Looking at who.is, his DNS records look correct and are pointed to my server. He's got a TTL set to 600s, and running through a variety of WHOIS tools, his domain resolves to my IP address.

Running nslookup from my server's shell it shows my server's ip in the non-authoritative answer, and localhost (127.0.0.1) as the server.

I'm running my network on MediaTemple, and have registered his domain as an alternate on my site, and I've created the symlinks (mt) requires for a mapped domain to work. Visiting his domain in a browser brings me to my network's wp-signup page.

The define('SUNRISE', on); line is in my wp-config.php file. Commenting out the 4 lines beginning with define( 'DOMAIN_CURRENT_SITE', 'mysite'); has no effect.

I'm lost here. Any suggestions of what I'm missing?