Do you have a robots.txt file set up on your server? Here’s what Google says in their Webmaster Guidelines:
-Make use of the robots.txt file on your web server. This file tells crawlers which directories can or cannot be crawled. Make sure it’s current for your site so that you don’t accidentally block the Googlebot crawler.
Visit http://www.robotstxt.org/wc/faq.html to learn how to instruct robots when they visit your site.