Then you should be able to access your site using client code. Use the ip address of the server (ifastnet.com) to connect, then put a "Host: mydomain.com" parameter in the request header. Here is the wifi shield example I use:
client.println("GET /mypage.php HTTP/1.1");
client.println("Host: mydomain.com");
client.println("Connection: close\r\n");
edit: Actually, if you have your domain name registered correctly, it should resolve to the ifastnet server anyway.