Here is the javascript code I use to create the array and send it on it's way:

<script type="text/javascript" language="javascript">
    $(document).ready(function () {
        $("#update-cart-btn").click(function() {
            var items = [];
            $(".item").each(function () {
                var productKey = $(this).find("input[name='item.ProductId']").val();
                var productQuantity = $(this).find("input[type='text']").val();
                items[productKey] = productQuantity;
            });

            $.ajax({
                type: "POST",
                url: "@Url.Action("UpdateCart", "Cart")",
                data: items,
                success: function () {
                    alert("Successfully updated your cart!");
                }
            });
        });
    });
</script>

The items object is properly constructed with the values I need.

What data type must my object be on the backend of my controller?

I tried this but the variable remains null and is not bound.

[Authorize]
[HttpPost]
public ActionResult UpdateCart(object[] items) // items remains null.
{

    // Some magic here.
    return RedirectToAction("Index");
}