For the last several preview releases, ASP.NET MVC features have been split across two assemblies - System.Web.Mvc.dll and Microsoft.Web.Mvc.dll.  The later assembly + namespace contains "futures" features that hadn't yet been committed to ship in the core V1 product.  As features become "committed" we move them from the futures assembly into the core assembly - and also change the namespace (from Microsoft.Web.Mvc to System.Web.Mvc). 

The previous preview releases automatically shipped and added the "futures" assembly when you did a File->New ASP.NET MVC project.  Starting with today's beta we are no longer automatically adding this assembly - instead you need to explicitly add it from your project if you want to use it.  The reason for this is so that developers can clearly distinguish those features that will be in the fully supported V1 product (which implies product support and a higher commitment around backwards compatibility), and those that might still evolve in the future (and not be added to the supported product until vnext).

Important: the futures assembly (along with all the source code in it) will continue to ship and will work with ASP.NET MVC V1.  So if there is a feature in it you really like, you do not have to worry about it disappearing on you (it is still there and you can still use it).  You just now need to explicitly reference the assembly and use it in your project. 

We plan to ship a version of the ASP.NET MVC Futures assembly that works with the Beta later today.  You will be able to download it here.