Official Google Webmaster Central Blog: Introducing Rich Snippets
http://googlewebmastercentral.blogspot.com/2009/05/introducing-rich-snippets.html

Now, time for some Q&A with the team: If I mark up my pages, does that guarantee I'll get Rich Snippets? No. We will be rolling this out gradually, and as always we will use our own algorithms and policies to determine relevant snippets for users' queries. We will use structured data when we are able to determine that it helps users find answers sooner. And because you're providing the data on your pages, you should anticipate that other websites and other tools (browsers, phones) might use this data as well. You can let us know that you're interested in participating by filling out this form. What about other existing microformats? Will you support other types of information besides reviews and people? Not every microformat corresponds to data that's useful to show in a search result, but we do plan to support more of the existing microformats and define RDFa equivalents. What's next?