This makes testing with domains and sub-domains easier on a mac, choose "Automatic Proxy Configuration" in Airport settings and set the URL to this file. Then any request to port 3000 will be pointed to the local rails server e.g. http://sub.mydomain.c

RailsProxy.pac

function FindProxyForURL(url, host) {

  var port = url.match(/^\w{3,5}:\/\/[^:\/]*(:(\d+))?/)[2];
  if(port){
    if(port=="3000"){
      var proxy = "PROXY localhost";
      if(port){proxy += ":" + port;}
      return proxy;
    }
  }
  return "DIRECT";
}