Nice trick. Spoiler:

  HTTP/1.1 200 OK
  Date: Mon, 19 Mar 2012 08:49:27 GMT
  Server: Apache
  Link: <_asdf.css>;rel="stylesheet";type="text/css";media="all"
  Content-Length: 0
  Connection: close
  Content-Type: text/html; charset=UTF-8

That one's fun - I thought it was going to be more complicated and never considered the method you used :)

Very cool.

With httpfox I got

html:after { padding-left: 5px; content: 'Can you view my source from Firefox or Opera?'; }

Edit: It's also possible to view the source with Firefox JSview, Web Developer addons and curl.