Problems implementing ValidatingAntiForgeryToken attribute for Web API with MVC 4 RC

I'm making JSON-based AJAX requests and, with MVC controllers have been very grateful to Phil Haack for his Preventing CSRF with AJAX and, Johan Driessen's Updated Anti-XSRF for MVC 4 RC. But, as I transition API-centric controllers to Web API, I'm hitting issues where the functionality between the two approaches is markedly different and I'm unable to transition the CSRF code.

ScottS raised a similar question recently which was answered by Darin Dimitrov. Darin's solution involves implementing an authorization filter which calls AntiForgery.Validate. Unfortunately, this code does not work for me (see next paragraph) and - honestly - is too advanced for me.

As I understand it, Phil's solution overcomes the problem with MVC AntiForgery when making JSON requests in the absence of a form element; the form element is assumed/expected by the AntiForgery.Validate method. I believe that this may be why I'm having problems with Darin's solution too. I receive an HttpAntiForgeryException "The required anti-forgery form field '__RequestVerificationToken' is not present". I am certain that the token is being POSTed (albeit in the header per Phil Haack's solution). Here's a snapshot of the client's call:

$token = $('input[name=""__RequestVerificationToken""]').val();
$.ajax({
    url:/api/states",
    type: "POST",
    dataType: "json",
    contentType: "application/json: charset=utf-8",
    headers: { __RequestVerificationToken: $token }
}).done(function (json) {
    ...
});

I tried a hack by mashing together Johan's solution with Darin's