var update_upload_status_x = true;
function update_upload_status(url)
{
  if (!update_upload_status_x)
  {
    update_upload_status_x = true;
    return true;
  }
  new Ajax.Updater('upload_progress', url, {
    parameters: { uid: $('UPLOAD_IDENTIFIER').value}, 
    evalScripts: true,
    onSuccess: function(){
      setTimeout('update_upload_status(\''+url+'\')', 300);
    }
  });
}