A very simple problem. (And probably a very simple answer)

I am trying to override the function theme_node_submitted(). I place the code below in my template.php file, but nothing happens. Any help is appreciated.

function phptemplate_node_submitted($node) {
  return t('This is a test... !username on @datetime',
    array(
      '!username' => theme('username', $node),
      '@datetime' => format_date($node->created, 'custom', 'M d, y'),
    ));
}