Microsoft has just released a 2 hour clinic titled "Developing Enhanced Web Experiences with Microsoft ASP.NET AJAX Extensions". This course offers an introduction and overview of ASP.NET Ajax architecture, and has some simple walkthrough examples of building ASP.NET Ajax applications, enhancing existing ASP.NET applications with ASP.NET Ajax, and calling web services and web methods from the client side.

Clinic 5230: Developing Enhanced Web Experiences with Microsoft ASP.NET Ajax Extensions

Did I mention already that it's free?