It appears that at the time there's no option for this but to hack the code. Here's how we added a custom label for our fivestar rating:
- Search for all instances of t('Average') in /modules/fivestar/fivestar.module
- Replace the following instances (not all of them) with a label of your choice:
line 1238:
switch ($settings['style']) {
case 'average':
$form['vote']['#title'] = t('Rate this article');
line 1248:
case 'smart':
$form['vote']['#title'] = $values['user'] ? t('Your rating') : t('Rate this article');
line 1269:
$form['average'] = array(
'#type' => 'markup',
'#value' => theme('fivestar_static_element', $static_average, $settings['title'] !== FALSE ? t('Rate this article') : NULL, $static_description),
Note that the line #'s may differ depending on the version of the module you're using. This was for // $Id: fivestar.module,v 1.13.2.72 2009/10/04 17:16:48 ezrag Exp $.
Hope that helps!