Historically, search engines have had trouble accessing AJAX-based content and this proposal would enable Google (and presumably other search engines that adopted the standard) to index more of the web. The standard SEO advice for AJAX implementations has traditionally been to follow accessibility best practices. With Google’s proposal, an AJAX-generated URL that contains a hash mark (#) would also be replaced with a URL that uses #! in place of #.
http://searchengineland.com/googles-proposal-for-crawling-ajax-may-be-live-34411