Just a small fix that helped me get running SWFUpload on one of my Drupal7 installations where some module has closing PHP tag or something like that and makes me some whitespace before output(which breaks SWFUpload - it is freezed at 95% of progress):

in swfupload.admin.inc around line 103 i added ob_clean();

  ob_clean();
  echo json_encode($p);
  exit(0);

maybe, this is duplicate/solution of #719604: Files are not uploaded