function no_link_current_page( $p ) {
return preg_replace( '%((current_page_item|current-cat)[^< ]+)[^>]+>([^< ]+)</a>%', '$1<span>$3</span>', $p, 1 );
}
add_filter( 'wp_list_pages', 'no_link_current_page' );
add_filter( 'wp_list_categories', 'no_link_current_page' );