Right, the idea would be to put the logic at the top of the main php file for wordpress, something like:

  # oversimplified, but...
  $key=md5(http_build_query($_GET));
  $content=apc_fetch($key,$success);
  if ($success === TRUE) {
      echo $content;
      return ;# never invoke wordpress
  }
  # wordpress here
  # some logic at the bottom to store what wordpress
  #   just rendered into the cache