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.

Check out ScottGu's Blog for more info.