server_name supports suffix matches using .mydomain.com syntax:
server {
server_name .mydomain.com;
rewrite ^ http://www.adifferentdomain.com$request_uri? permanent;
}
or on any version 0.9.1 or higher:
server {
server_name .mydomain.com;
return 301 http://www.adifferentdomain.com$request_uri;
}