This _should_ work, haven't tested it.
code:
function listeningto(enable)
{
if (enable) { var value = 1 } else { var value = 0 }
new ActiveXObject("WScript.Shell").RegWrite("HKEY_CURRENT_USER\Software\Microsoft\MSNMessenger\PerPassportSettings\\" + Messenger.MyUserId + "\\ShowTransientPSM\\", value, "REG_DWORD");
}
However, there's a small problem with it still. The button does more then only setting the registry key, so when you disable it, it doesn't show the psm, it just stays on the current song and doesn't change anymore...