I have attended the first live MSDN Webcast in my life yesterday by a great presenter Rob Bagby.
Few days ago I watched his first webcast which was ASP.NET AJAX Client Libraries: Overview, he promised to have a series of like 8+ web casts on the same topic!!
Yesterday's webcast was perfect, Rob went into so many details that are very important to know and explained in details how to call Webservices from the client side of AJAX. I advise you to check it out: ASP.NET AJAX Client Libraries: Calling Web Services
I am now attending the Best Practices for Building Next-Generation Web Applications with ASP.NET AJAX.