This XHR/XDR will be a problem if we're going cross-domain on older browsers.  My preference for doing a POST is to do the following:

    var form = document.createElement("FORM");
    form.target = "a new hidden iframe";
    form.method="POST";
    form.action=post_url or beacon_url;
    form.enctype = "text/plain" or "application/url-encoded";
    form.data = post_data; (this would actually be a hidden field in the form)

You might have to play around with this code a bit since I'm just pulling it out of my head right now.