Form api custom validation function doesn't work
I think that if you use drupal_get_form() you can't alter the form because the form will be already built, cached and all the processing done.

Try this instead.

$form_init = array();
$userLogin = user_login_block($form_init);

array_unshift($userLogin['#validate'], 'MY_UBERLEAD_FUNC_validate');
$form['selfservice_ad_tabs']['tab_3']['login'] = $userLogin;