Eilon was correct – they made it easier!

It's trivial to test TempData now. This is all that's needed to port the example from his earlier post to Preview 4 (thanks Ben):

[TestFixture]
public class HomeControllerTests
{
    [Test]
    public void IndexSavesUserIDToTempDataAndRedirects()
    {
        var homeController = new HomeController();
        homeController.TempData = new TempDataDictionary();

        var result = homeController.Index();

        Assert.That(result.Values.ContainsKey("action"));
        Assert.That(result.Values["action"], Is.EqualTo("GreetUser"));
        Assert.That(homeController.TempData.ContainsKey("UserID"));
        Assert.That(homeController.TempData["UserID"], Is.EqualTo("user123"));
    }

    [Test]
    public void GreetUserWithNoUserIDRedirects()
    {
        var homeController = new HomeController();
        homeController.TempData = new TempDataDictionary();

        var result = homeController.GreetUser() as RedirectToRouteResult;

        Assert.That(result, Is.Not.Null);
        Assert.That(result.Values.ContainsKey("action"));
        Assert.That(result.Values["action"], Is.EqualTo("ErrorPage"));
        Assert.That(homeController.TempData, Is.Empty);
    }

    [Test]
    public void GreetUserWithUserIDCopiesToViewDataAndRenders()
    {
        var homeController = new HomeController();
        homeController.TempData = new TempDataDictionary();

        homeController.TempData["UserID"] = "TestUserID";

        var result = homeController.GreetUser() as ViewResult;

        Assert.That(result, Is.Not.Null);
        Assert.That(result.ViewName, Is.EqualTo("GreetUser"));
        Assert.That(result.ViewData, Is.Not.Null);
        Assert.That(result.ViewData.ContainsKey("NewUserID"));
        Assert.That(result.ViewData["NewUserID"], Is.EqualTo("TestUserID"));
    }
}

If you actually need to test TempData, you can implement a stub ITempDataProvider and set the TempDataProvider propery on the controller:

public class TestTempDataProvider : ITempDataProvider
{
    public TempDataDictionary TempDataDictionary { get; private set; }

    public TempDataDictionary LoadTempData()
    {
        return TempDataDictionary;
    }

    public void SaveTempData(TempDataDictionary tempDataDictionary)
    {
        TempDataDictionary = tempDataDictionary;
    }
}

(Edited 6/17 with the proper usage from Ben)