It's more a "web2.0" enchantment than a bug that concerns AJAX and Browser history.
If you open a member's profile, e.g. ejm and hit his Postings tab, wait for the ajax request to complete and click on a random post of him (you mustn't open a new browser tab or window), then click the back button (or backspace) inside your browser, you get the previous page but the Postings history gets erased and you have to redo the ajax request.
This is the desired functionality (Please add dynamic title changes, so we can see where we were exactly in history and scroll positions that will also fix the IE bug!!!):