According to a post at googlewebmaster blog Michael Wyszomierski and Greg Grothaus, Search Quality has mentioned, "Many webmasters have discovered the advantages of using Ajax to improve the user experience on their sites, creating dynamic pages that act as powerful web applications. But, like Flash, Ajax can make a site difficult for search engines to index if the technology is not implemented carefully." Read more- http://googlewebmastercentral.blogspot.com/2007/11/spiders-view-of-web-20.html