Works for me...

  > curl -i 'httpstat.us/418'
  HTTP/1.1 418 Unknown Error
  Server: nginx
  Date: Wed, 25 Jan 2012 23:58:13 GMT
  Content-Type: text/plain
  Connection: keep-alive
  Access-Control-Allow-Origin: *
  Content-Length: 16
  X-Varnish: 1346742606
  Age: 0
  Via: 1.1 varnish

  418 I'm a Teapot

although it shows as 'Unknown Error' instead of 'I'm a Teapot'