pressflow compatibility.

@voxpelli not really, the backup is only used to restore the session, and user, the calls used to do so are benign when used with null data. see services.module lines 870-894

function services_session_unload($backup) {
  global $user;

  // No point in reverting if it's the same user's data
  if ($user->sid == $backup->sid) {
    return;
  }

  // Some client/servers, like XMLRPC, do not handle cookies, so imitate it to make sess_read() function try to look for user,
  // instead of just loading anonymous user :).
  if (!isset($_COOKIE[session_name()])) $_COOKIE[session_name()] = $backup->sessid;

  // Save current session data
  sess_write($user->sid, session_encode());

  // Empty current session data
  $_SESSION = array();

  // Revert to previous user and session data
  $user = $backup;
  session_id($backup->sessid);
  session_decode($user->session);

  session_save_session(TRUE);
}

there is no argument that the backup is triggered, but there is no need to change the way the backup is restored, the change made in the patch doesn't completely remove the backup it just removes the session from the backup when there isn't a session to backup. when the backup is restored it will set the session to the same state it was when the backup was created. which is non-existent.
which is planned behavior,

session_decode does not throw an error when null is passed to it.