I have several views that use different view data types. However, they all use the same master page. I thought I could derive the different view data types from a base class, and then derive the master from that same base type, but the compiler doesn't like it.

 I am I using the wrong approach here? I could use an interface but then all the different view data types have to implement the same functionality. I'd like to use a base class to share the functionality.

I was expecting that to have been fixed in preview #2 (see http://forums.asp.net/t/1195178.aspx for CTP #1 workaround).

Looking at the source for ViewMasterPage<ViewData> (from http://codeplex.com/aspnet): 

public class ViewMasterPage<TViewData> : ViewMasterPage {
  public new TViewData ViewData {
    get {
      ViewPage<TViewData> viewPage = Page as ViewPage<TViewData>;
      if (viewPage == null) {
        throw new InvalidOperationException(String.Format(CultureInfo.CurrentUICulture, MvcResources.ViewMasterPage_RequiresViewPageTViewData));
      }
      return viewPage.ViewData;
    }
  }
}

This has been rewritten, but still does not handle the case where the type parameter of the master page is a base class of the type parameter of the page.

I can't understand why there is an attempt to case the page, I would have expected a direct cast of the ViewData member of the ViewContext:

public class ViewMasterPage<TViewData> : ViewMasterPage {
  public new TViewData ViewData {
    get {
      TViewData viewData = ViewContext.ViewData as TViewData;
      if (viewData == null) {
        throw new InvalidOperationException(String.Format(CultureInfo.CurrentUICulture, MvcResources.ViewMasterPage_RequiresViewPageTViewData));
      }
      return viewData;
    }
  }
}

You can either (a) modify the source and compile your own System.Web.Mvc.dll, or (b) create a shim class like in the thread above that fixes the ViewData property.