The IE team has recently announced that they have made a few AJAX improvements in the newest version of Internet Explorer. One of such improvements is support for updating the navigation log through AJAX. So when your application uses intensively AJAX to enhance the User Experience, you can still leverage the native browser navigation mechanism by updating the navigation log information.
It's definitely great improvement of usability and accessibility. What concerns me is the fact, that it will be supported in IE8 only. Making it cross-browser available would be definitely the way to go - especially for those who want to benefit of the rich AJAX experience on the Internet. I wonder how long will it take for other browser manufacturers to follow.