If you are still using the original ASP.NET AJAX 1.0 release, you may be looking for a way to use .NET 3.5 SP1′s “history” feature of the ScriptManager control.
A while back, Nikhil Kothari wrote a component called UpdateHistory, which looks like what was integrated into 3.5 SP1. It certainly solved my problem.
Combine it with HttpUtility.ParseQueryString and you’re onto a winner!