Except for those few platforms that automatically provide (very) limited AJAX interactions out of the box (Ruby on Rails, Lotus Domino 8.5+ with xPages, some JSF implementations, etc.), the back-end platform really doesn't matter. As long as it is capable of accepting an HTTP request (the full REST suite is desirable, but GET and POST are sufficient), processing that request, and returning a response (XML or JSON with the appropriate MIME type are normal, but even an HTML page will do the trick) with the desired information, you're good to go.
ASP, ASPX, PHP, RoR, CF, JSP and Domino 6+ -- along with any reasonably modern (say post-1997) CGI scripting environment -- can easily handle the server-side part of the AJAX equation. Heck, there are even database platforms out there, like CouchDB, that can directly handle AJAX requests and return data formatted as JSON without (necessarily) having to go through a separate scripting environment.
The hard part always has been the browser side, and current JS libraries like JQuery make that a whole lot easier than it has been historically.
If you are hand-rolling a solution, just make sure to practice safe sex -- make sure that both the browser-side and the server-side code can handle faults with a bit of grace and aplomb.
... i think this :s ... is more easy 