Microsoft released a preview of the ASP.NET 3.5 Extensions today. This release has new ASP.NET Ajax improvements, ASP.NET Dynamic Data support, new controls for Silverlight and the new ASP.NET MVC (Model View Controller) which can be downloaded here.
If you would like to get an overview of ASP.NET MVC you can check out Scott Gu's blog (ASP.NET MVC Framework).
Cheers,
Carlos