Yesterday I have done a javascript that calls a list of urls and returns its Status and Status Code. It uses XMLHttpRequest not ASP.NET AJAX :D
It's an easy .ascx control so you can easyly use and change it.
You can configure the pages that you want to test with code like this:
AjaxPageTester1.UrlList.Add("/default.aspx");
AjaxPageTester1.UrlList.Add("/login.aspx");
Download the code.