Hijax:

DOM Scripting: Hijax
Adactio: Journal - Progressive enhancement with Ajax

I like this idea of "progressive enhancement and graceful degradation" using a modular server-side architecture "capable of returning entire pages or portions of pages"! It's clear and doable!
Basically, you need to write a set of functions or maybe a class with a set of functions running on a server and then have one page [with server side script and no html] which handles a normal form POSTs using a set of those functions, and other pages which handle xmlhttp request post/get submissions by using an appropriate subset of the same functions. It could be even the same page which behaves differently depending on who and how calls it. A kind of Web Service :)
This page then either redirects to an appropriate HTML page, or just sends an appropriate XML or text back to AJAX caller.