With all the AJAX excitement, it's worth pointing out a simple technique I call JAH - which can be Just Async HTML, or Javascript Async HTML to taste.
This works on the microformat principle that XHTML is XML, but with the added advantage that Javascript already knows how to handle XHTML DOM's so no xml parsing is required.
You just include <script language="javascript1.3" src="jah.js" ></script> in the <head> and then link to dynamic pages with <a href="javascript:jah('kevin.html',' where target');">kevin</a>target is the id of the HTML element you want to replace.