Jack Slocum has made a start with 'ajaxification' of the well known forum software phpBB. He is using the Yahoo! UI Library (YUI) with Yahoo.ext, an extension to the Yahoo! UI Library. The results so far are impressive! Navigation works like a charm and is very user friendly.
Definitely one of the better examples of Ajax use. Not just fancy widgets, but actual an improved userinterface. Of course there's work to be done like one of the forum visitors mentioned:
And one thing to consider with your new layout is this... how will people be able to bookmark a specific topic so they can return to it?
I'm sure there will be a good solution for that. Let's keep an eye on how things develop!