Adds a visual editor to the description field for custom taxonomies. Example below will display the tinymce editor in every custom taxonomy description right now. Can edit to show it for only some specific taxonomy. See also - http://wordpress.org/plugins/rich-text-tags/

tax_visual_editor

/**
 * Display advanced TinyMCE editor in taxonomy page
 */
function wpse_7156_enqueue_category() {
    global $pagenow, $current_screen;

    if( $pagenow == 'edit-tags.php' ) {
        require_once(ABSPATH . 'wp-admin/includes/post.php');
        require_once(ABSPATH . 'wp-admin/includes/template.php');

        wp_tiny_mce( false, array( 'editor_selector' => 'description', 'elements' => 'description', 'mode' => 'exact' ));
    } 
}
add_action( 'init', 'wpse_7156_enqueue_category' );