I just discovered pjax and I find it completly awesome. If I understand it correctly, it's AJAX, but without its problems (fully degradable, the URL and the title of the page is changing when using it, compatible with search bots...).
But I am very curious as how it's working ? How can we change the URL in the browser, without the page to change... I read somewhere you need Firefox 4, is that a new "web" feature then ?