WordPress MU

I put the following code in /mu-plugins/

add_filter('the_content_rss','say_hello_word');
function say_hello_world($string) { return 'hello world'; }

It didn't have any affect on the RSS feeds. I adjusted the code as follows:

add_filter('the_content','say_hello_word');
function say_hello_world($string)
{
  if(is_feed()) { return 'hello world'; }
  else { return $string; }
}

This does work but it's not being applied to every post in the RSS feeds.

Bump... any ideas?