I've been using ajax for a while now, and a real problem is that the browser caches the xml like crazy, so if your not using parameters, then getting the most up to date xml can be a problem.
I looked around and found some good suggestions
use a server side script to output the xml + a no-cache header
add a custom header to the xml type on the web server (usually not an option)
add a 'get if newer than' header to your request (set newer than = jan 1 1970)
add an arbitrary random parameter to your request
Solution 4 is brilliant and simple, so just add a random guid to your request! eg :
url += "?sid="+Math.Random;