Visual Studio 2010 Beta 2 is recently made generally available to try and it has ASP.NET MVC2 Preview 2 out of box. In ASP.NET MVC2; there is a breaking change from ASP.NET MVC 1. The ASP.NET MVC’ built in JSON support will only serve for POST request and discards GET requests due to JSON Vulnerability. Michael Campbell has written a detailed article on DevProConnections. The article and the links it has; are must read for AJAX developers using JSON.
You don't need Visual Studio 2010 Beta 2 to try ASP.NET MVC2; ASP.NET MVC2 is side by side release and you can use both ASP.NET MVC1 and ASP.NET MVC2 with Visual Studio 2008.