I am trying to utilize the History API on a rather high profile project. At http://diveintohtml5.info/history.html Mark Pilgrim links to an example he created to show how it works. The example is at: http://diveintohtml5.info/examples/history/fer.html In the example when you use the "Previous" and "Next" buttons in the image gallery it brings up a picture of a different dog and the URL in the address bar makes it appear to the user that the browser has advanced a page. In reality the History API is just bringing in photo and caption information for a different dog while keeping the rest of the page content the same.
View here: http://www.cmzmedia.com/fer.html (only has one "Next" example to work with for now)