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You can use form_state instead of form in your submit handler

function module_simplenews_unsubscribe($form, &$form_state) {

  if ($form_state['values']['checkbox'] == 0) {
    drupal_set_message(t("You are checked"));
  }
 if ($form_state['values']['checkbox'] == 1) {

    drupal_set_message(t("You are unchecked"));
  }

Use debug tools for proper index of form_state for checkbox.