MVC isn't generating the client-side validation rules for my viewmodel. The HTML just contains this:

<script type="text/javascript">
//<![CDATA[
if (!window.mvcClientValidationMetadata) { window.mvcClientValidationMetadata = []; }
window.mvcClientValidationMetadata.push({"Fields":[],"FormId":"form0","ReplaceValidationSummary":false});
//]]>
</script>

Note that Fields[] is empty!

My view is strongly-typed and uses the new strongly-typed HTML helpers (TextBoxFor(), etc).

View Model / Domain Model

public class ItemFormViewModel
{
    public Item Item { get; set; }
    [Required] [StringLength(100)] public string Whatever { get; set; } // for demo
}
[MetadataType(typeof(ItemMetadata))]
public class Item
{
    public string Name { get; set; }
    public string SKU { get; set; }
    public int QuantityRequired { get; set; }
    // etc.
}
public class ItemMetadata
{
    [Required] [StringLength(100)] public string Name { get; set; }
    [Required] [StringLength(50)] public string SKU { get; set; }
    [Range(0, Int32.MaxValue)] public int QuantityRequired { get; set; }
    // etc.
}

(I know I'm using a domain model as my / as part of my view model, which isn't a good practice, but disregard that for now.)

View

<%@ Page Language="C#" MasterPageFile="~/Views/Shared/Site.Master" 
Inherits="System.Web.Mvc.ViewPage<ItemFormViewModel>" %>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <h2>Editing item: <%= Html.Encode(Model.Item.Name) %></h2>

    <% Html.EnableClientValidation(); %>
    <%= Html.ValidationSummary("Could not save the item.") %>

    <% using (Html.BeginForm()) { %>

        <%= Html.TextBoxFor(model => model.Item.Name) %>
        <%= Html.TextBoxFor(model => model.Item.SKU) %>
        <%= Html.TextBoxFor(model => model.Item.QuantityRequired) %>

        <%= Html.HiddenFor(model => model.Item.ItemID) %>

        <%= Html.TextBox("Whatever", Model.Whatever) %>
    <input type="submit" value="Save" />
    <% } %>
</asp:Content>

I included the Whatever property on the view model because I suspected that MVC wasn't recursively inspecting the sub-properties of ItemFormViewModel.Item, but even that isn't being validated? I've even tried delving into the MVC framework source code but have come up empty. What could be going on?