I think you're a little confused about what AJAX technologies are and where they run.

AJAX is a presentation philosophy (AKA: a client-side issue). It runs independent of the server technology used. On various projects, I have implemented AJAX on servers running PHP, ColdFusion, and static HTML. AJAX is server platform independent.

As for the particulars of IE7, I can say that using script.aculo.us and Prototype libraries run the same if not improved on IE7 in comparison to IE6. The fact that the libraries themselves are actively being tested for IE7 as new beta comes out means that I don't have to do anything extra for the changes; It just works.

I understand the initial concern for IE7/IE6 compatibility, but sticking with a popular library solve this problem and make the concern a non-issue.

As for the server-side of AJAX, what you'll be coding are pages that output either HTML, XML, or JSON. Any server platform can create this kind of output, so questions of server compatibility are moot.

But my word of cation is this: Know why you are changing a component to an AJAX philosophy and how best to implement it. There are good reasons to use AJAX as there are bad ones. Please proceed with cation and purpose.