A rich snippet example from Schema.org http://schema.org/AggregateRating:
<html>
<div itemscope itemtype="http://schema.org/Product">
<img itemprop="image" src="dell-30in-lcd.jpg" />
<span itemprop="name">Dell UltraSharp 30" LCD Monitor</span>
<div itemprop="aggregateRating"
itemscope itemtype="http://schema.org/AggregateRating">
<span itemprop="ratingValue">87</span>
out of <span itemprop="bestRating">100</span>
based on <span itemprop="ratingCount">24</span> user ratings
</div>
</div>
</html>
But http://www.google.com/webmasters/tools/richsnippets won't show a preview.
So, the following words from http://support.google.com/webmasters/bin/answer.py?hl=en&answer=146645 are just lies?
New! schema.org lets you mark up a much wider range of item types on your pages, using a vocabulary that Google, Microsoft, and Yahoo! can all understand. Find out more. (Google still supports your existing rich snippets markup, though.)