C# - MVC 2 - Force User To Select A Value For A Non-nullable Type?

I have (or so I think!) a simple problem.

I will greatly simplify my model to speed this along.

I have a model class lets call it item:

public class Item
{
[Required(ErrorMessage="Must indicate if product is to be tracked by serial number.")]
public bool TrackedBySerial { get; set; }
}

I have an "Add" view where I create a DropDownList like so:

<%= Html.DropDownListFor(model=>model.TrackedBySerial, new SelectList(new List<object> {null,true,false},null),"Select One") %>
<%= Html.ValidationMessageFor(model => model.TrackedBySerial) %>

My problem is that unless I create my boolean value in the model to be a nullable type, I can't force the blank default value.

If I use the Html.DropDownList() instead of DropDownListFor() , is there any way for me to use ModelState.IsValid -- or do I need to mix my own custom validation in my controller as well?

Update: So I got the functionality I was looking for, its just a bit more verbose than I'd have liked. Is there a better way to do this?

Controller:

[HttpPost]
public ActionResult Add(InventoryItem newItem)
{
try
{
//get the selected form value
string formVal = Request.Form["TrackBySerial"];
//convert to true, false, or null
bool? selectedValue = TryParseNullable.TryParseNullableBool(formVal);
//if there is no value, add an error
if (!selectedValue.HasValue).....

view:

<div class="editor-field">
<%=Html.DropDownList("TrackBySerial","- Select One- ") %>
<%=Html.ValidationMessage("TrackBySerial") %>
<%= Html.ValidationMessageFor(model => model.TrackedBySerial) %>
</div>