Cross-site AJAX requests

var serviceUrl = "http://myservicedomain";
var payload = "<myRequest><content>Some content</content></myRequest>";
var request = new XMLHttpRequest();
request.open("POST", serviceUrl, true); // <-- This fails in Mozilla Firefox amongst other browsers
request.setRequestHeader("Content-type", "text/xml");
request.send(payload);

function ajaxWorkaroung() {
    var frm = gewtElementById("myIFrame")
    frm.src = "http://some_other_domain"
}
function ajaxCallback(parameter){
    // this function will be called from myIFrame's content
}