The current plan is for Html.RenderAction() to be moved out of Futures and into the core product as of MVC 2 Beta. (Plans are always subject to change, obviously.)
VS2008 SP1:
MVC 2 Beta will target VS2008 SP1. When MVC 2 is complete, it will continue to target VS2008 SP1.
VS2010:
VS2010 Beta 2 ships with MVC 2 Preview 2, which is an earlier release than MVC 2 Beta and thus does not include Html.RenderAction(). If you need it for this build, you can find it in MVC 2 Preview 2 Futures. When VS2010 is complete, it will ship with MVC 2 RTM.
ASP.NET 4 / .NET 4
MVC 2 is technically an out-of-band release, so even though it will ship with VS2010, it's not part of ASP.NET 4 or .NET 4. That is, you shouldn't expect System.Web.Mvc.dll to be a part of any .NET 4 installer. It will be able to be downloaded separately after release.