Does the externally facing DNS server resolve to the correct IP address?
The ip isn't 10.x.x.x or 192.168.x.x is it? Those are non-routable over the internet..
Is there a firewall in the way?
Can you ping www.mydomain.com
from the outside?
Can you do:
and connect to the port from the outside?
Does /var/log/httpd/access_log show any lines when you try to hit it?