There's been some trouble with the young technical darling called AJAX. When the state of the page is coded into the URL by using a hash mark (#) it works for the human visitor - but search engines do not consider the # mark to indicate a different url, because that character is also used commonly as a page fragment identifier.
Google's proposal is to insert a new token after the hash mark. That would alert crawling technology that the following information creates a new page state, often by going to the server to update only part of the page content.
The new token Google proposes is an exclamation mark added immediately after the hash tag, so a stateful AJAX url might look like this: