While AJAX-based websites are popular with users, search engines traditionally are not able to access any of the content on them. Google is currently working on a proposal and a prototype implementation for making AJAX-based websites crawlable to benefit webmasters and users by making content from rich and interactive AJAX-based websites universally accessible through search results on any search engine that chooses to take part. We believe that making this content available for crawling and indexing could significantly improve the web.

More info: Google Webmaster blog