How to Remove Subscribers from the Post Author Drop Down

I have a WP Multisite installation. On the main blog all subscribers are appearing in the post author dropdown on the post edit screen. I think this happened after an upgrade to 3.0.1.

The new user role is set to Subscriber, so it’s not like all the new signups are being made contributors or authors. Does anyone know if the problem I’m experiencing is a bug? Or am I missing something really obvious? We have hundreds of subscribers so it’s really a pain to try to find the right one to assign a post author.

Any help / hacks would be much appreciated. Thank you.

ALSO…. when I look at edit-form-advanced.php , I see this:

if ( post_type_supports($post_type, 'author') ) {
    $authors = get_editable_user_ids( $current_user->id ); // TODO: ROLE SYSTEM
    if ( $post->post_author && !in_array($post->post_author, $authors) )
        $authors[] = $post->post_author;
    if ( ( $authors && count( $authors ) > 1 ) || is_super_admin() )
        add_meta_box('authordiv', __('Author'), 'post_author_meta_box', $post_type, 'normal', 'core');
}

Is there anything I can change in there to fix this?