I don't know why, but actually the function can only show one, more than one or all categories. It could be useful to add the exclusions of one or more categories, because when it is not possible to use query_posts (out of the loop) it's really useful to have this function.
Regards,
Alessio Periloso
PS: Actually I've hacked wp-includes/post.php to exlude one given category, I'm looking for help to exclude more categories.