As I mentioned last week when ASP.NET AJAX 1.0 shipped, we are publishing the full source code to the ASP.NET AJAX product. This includes the source to the server-side ASP.NET integration (including the UpdatePanel, UpdateProgress, and ScriptManager controls, as well as the source to the Network Serialization code).
The code for the server-side ASP.NET AJAX 1.0 implementation was released this morning. You can download it here. It is being released under the Microsoft Reference License (Ms-RL). Included with the source code are debugger symbols for the shipped binary, which will allow you to step from your own code into the ASP. NET AJAX library while debugging, with line number and symbol data preserved. Note that the setup installs the source code locally on your machine within the "\Program Files\Microsoft ASP.NET\ASP.NET 2.0 AJAX Extensions\v1.0.61025\Source" directory.
You can also obviously download (and modify) the source code for the ASP.NET AJAX Control Toolkit. It is built as a collaborative CodePlex Project that both Microsoft and non-Microsoft developers contribute code and work on together.
P.S. I'm hoping on a plane to Europe in an hour for my presentations in Belgium and the UK, and will be out the rest of this week (it only took me 3 frantic hours to find my passport last night -- sheesh). As a result there will be some delays with blog comments (and my responses to them) while I'm away.