ASP.NET MVC 2 - Html.DropDownListFor confusion with ViewModel

I'm getting totally lost and confused on how to use the new strongly typed Html.DropDownListFor helper on ASP.NET MVC 2.0 R2

In the View I'm writting:

<%= Html.DropDownListFor(m => m.ParentCategory, new SelectList(Model.Categories, "CategoryId", "Name", Model.ParentCategory), "[ None ]")%>

<%= Html.ValidationMessageFor(m => m.ParentCategory)%>

and my Model object is thus:

public class CategoryForm : FormModelBase
{
    public CategoryForm()
    {
        Categories = new List<Category>();

        Categories.Add(new CategoryForm.Category() {
                           CategoryId = 1, 
                           Name = "CPUs" });
        Categories.Add(new CategoryForm.Category() { 
                           CategoryId = 2, 
                           Name = "Memory" });
        Categories.Add(new CategoryForm.Category() { 
                           CategoryId = 3, 
                           Name = "Hard drives" });
    }

    // ...other props, snip... //

    public Category ParentCategory { get; set; }

    public IList<Category> Categories { get; protected set; }

    public class Category
    {
        public int? CategoryId { get; set; }
        public string Name { get; set; }
    }
}

The problem is that when I select an item from the dropdown list, say the first item, I get the following ValidationMessageFor error "The value '1' is invalid."

So I change the View to...

<%= Html.DropDownListFor(m => m.ParentCategory.**CategoryId**, 
                              new SelectList .../ snip  ) %>

Now it works, kinda. The ParentCategory property in my ViewModel is set with the correct 'CategoryId' but the 'Name' is NULL. Am I better off just having a nullable int for ParentCategory property instead of a strongly typed 'Category' object?