The IE8 team have put up a post confirming that the Back/Forward buttons can be used in IE8 with an AJAX application. No more hitting back and watching the app get reset to the start. It's great to see this feature added. I've used IE8 beta for a while now and I must say I do like it. Looking forward to getting my hands on beta 2.
One of the AJAX improvements we adopted in IE8 from HTML5 is AJAX page navigations. In IE8 mode, we provide support for script to update the travel log components (for e.g. back/forward buttons, address bar) to reflect client-side updates to documents. This allows a better user experience where users can navigate back and forth without messing the AJAX application state.