The version of AJAX in ASP.NET 3.5 is 1.0.  AJAX 1.0 shipped as a separate download that you could install on top of ASP.NET 2.0.  Starting with the .NET Framework 3.5 release, all of these features are built-in with ASP.NET, which means you no longer have to download and install a separate ASP.NET AJAX setup when building or deploying applications.