Eilon Lipton, the developer for UpdatePanel recently posted some great resources to the forums that are a must read for anyone interested in UpdatePanel (yes all of you :))
What's up with UpdatePanels and how come nothing works?
This is a higher level piece that explains how UpdatePanel changed from being automatic (and broken) in the CTP to the new functionality in the beta.
HOWTO: Write controls compatible with UpdatePanel without linking to the ASP.NET AJAX DLL
This article goes in depth on how to use the new registration APIs on ScriptManager, but without linking to the Atlas assembly