Omega template.php form alter
Sorry I checked your code again and found the answer , change your code to this :

function omega_starter_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form['#id'] == 'user_login_form') { //form ID was changed
    //dpm($form);
    $form['actions']['submit']['#value'] = t("Custom login button!");
  }
}