On Dec 9, Microsoft released the ASP.NET 3.5 Extensions CTP Preview for Visual Studio 2008. Scott Guthrie outlines some ASP.NET 3.5 Extensions CTP Preview key features this preview release provides.
ASP.NET Ajax Improvements
ASP.NET Dynamic Data Support
ASP.NET Silverlight Support
ADO.NET Data Services
A set of ASP.NET 3.5 quickstart samples are also available.
A quick take:
Q: Should I use this for production development?
A: No, this is a CTP Preivew. See the ASP.NET Product Roadmap for more information.
Q: If I shouldn't use this for production development, what good is this release?
A: It provides the ASP.NET MVC which if your requirements need it provide the ability for enhanced URL control for your ASP.NET applications. Previously, the only ways to do a very complex URL structure was a painstaking folder path, or installing an Isapi extension.