Exist any way to integrate Google Rich Snippets in D6?
Given the comments, I am thinking all you'd need to do is theme the fivestar summary with google's snippets.  To do that, I pulled the theme_fivestar_summary() out of the module and plopped it into my theme's template.php file like so:

function YOURTHEME_fivestar_summary($user_rating, $average_rating, $votes, $stars = 5, $feedback = TRUE) {
  $output = '&lt;span itemprop="aggregateRating" itemscope itemtype="http://schema.org/AggregateRating"&gt;'; // ADDITION #1
  $div_class = '';
  if (isset($user_rating)) {
    $div_class = isset($votes) ? 'user-count' : 'user';
    $user_stars = round(($user_rating * $stars) / 100, 1);
    $output .= '&lt;span class="user-rating"&gt;'. t('Your rating: &lt;span&gt;!stars&lt;/span&gt;', array('!stars' =&gt; $user_rating ? $user_stars : t('None'))) .'&lt;/span&gt;';
  }
  if (isset($user_rating) &amp;&amp; isset($average_rating)) {
    $output .= ' ';
  }
  if (isset($average_rating)) {
    $div_class = isset($votes) ? 'average-count' : 'average';
    $average_stars = round(($average_rating * $stars) / 100, 1);
    $output .= '&lt;span class="average-rating"&gt;'. t('Average: &lt;span itemprop="ratingValue"&gt;!stars&lt;/span&gt;', array('!stars' =&gt; $average_stars)) .'&lt;/span&gt;'; // ADDITION #2
  }
  if (isset($user_rating) &amp;&amp; isset($average_rating)) {
    $div_class = 'combo';
  }

  if (isset($votes) &amp;&amp; !(isset($user_rating) || isset($average_rating))) {
    $output .= ' &lt;span class="total-votes"&gt;'. format_plural($votes, '&lt;span itemprop="reviewCount"&gt;@count&lt;/span&gt; vote', '&lt;span itemprop="reviewCount"&gt;@count&lt;/span&gt; votes') .'&lt;/span&gt;'; // ADDITION #3
    $div_class = 'count';
  }
  elseif (isset($votes)) {
    $output .= ' &lt;span class="total-votes"&gt;('. format_plural($votes, '&lt;span itemprop="reviewCount"&gt;@count&lt;/span&gt; vote', '&lt;span itemprop="reviewCount" &gt;@count&lt;/span&gt; votes') .')&lt;/span&gt;'; // ADDITION #4
  }

  if ($votes === 0) {
    $output = '&lt;span class="empty"&gt;'. t('No votes yet') .'&lt;/span&gt;';
  }

  $output = '&lt;div class="fivestar-summary fivestar-summary-'. $div_class . ($feedback ? ' fivestar-feedback-enabled' : '') .'"&gt;'. $output .'&lt;/div&gt;';
  $output .= '&lt;/span&gt;'; // ADDITION #5
  return $output;
}


and commented where I added the snippet additions.