see http://files.vbulletin.com/3.5/ajax.html
pretty cool, eh?
I've seen a CMS implementing Ajax, it's not matured
yet. Doesn't only look great but improves usability a
big deal.
Background infos:
http://en.wikipedia.org/wiki/AJAX
http://www.onlamp.com/pub/a/onlamp/2005/05/19/xmlhttprequest.html
http://www.ajaxian.com/
http://www.adaptivepath.com/publications/essays/archives/000385.php
http://www.google.com/webhp?complete=1&hl=en