Is Request.IsAjaxRequest() broken in the MVC RC1 refresh

<% using (Ajax.BeginForm("Contact", "AboutUs", new AjaxOptions()
      {
          OnBegin = "submitComments_begin",
          OnSuccess = "submitComments_success",
          OnFailure = "submitComments_failure",
          OnComplete = "submitComments_complete",
          LoadingElementId = "submitting"

      }, new { id="fooForm" }))

Request["__MVCASYNCPOST"] = "true"

Request.IsAjaxRequest()

public static bool IsAjaxRequest2(this HttpRequestBase request)
        {
            return !string.IsNullOrEmpty(
                   request.Params["__MVCASYNCPOST"] as string);
        }