"Create an AJAX request to the location (eg.: /about.html). Change the URL in the browser with the history API (this way, the back and forward buttons still work in the browser). Show a loading animation that the content is now being loaded. When the content is loaded, parse it to extract the contents of #main (you can help yourself there by adding markers in your HTML) and replace the content of your current #main section with the one you just loaded. Make sure that you handle all the links in your new #main content so they will act the same and fire off any BrowserScript required for the page that just loaded."
Aside of usability issues (what if the user has JavaScript disabled?), I don't really get how search engine spiders will be able to index the web site.