Installed ASP.Net 3.5 Extensions Preview today to use in a project and had some fun working out where the dlls were now living.
Finally found them in C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\ASP.NET 3.5 Extensions
Also remember, after switching your System.Web.Extensions and System.Web.Extensions.Design dll’s to the new version to change the references in your web.config from
System.Web.Extensions, Version=3.5.0.0 to System.Web.Extensions, Version=3.6.0.0