Checking what user a migration is running as
In the interest of trying to up the percentage of Drupal Questions that have an answer, I'll post the solution that I came to as a result of Clive's comment.

I added a preImport and postImport method to my migration class as follows:

  public function preImport() {
    parent::preImport();
    $GLOBALS['user']-&gt;previousid = $GLOBALS['user']-&gt;uid;
    $GLOBALS['user']-&gt;uid = 1;
    if ($GLOBALS['user']-&gt;uid != 1) {
      throw new MigrateException('Run Staff Contributor Migration as user 1.');
    }
  }

  public function postImport() {
    parent::postImport();
    $GLOBALS['user']-&gt;uid = $GLOBALS['user']-&gt;previousid;
  }


This ensures that this particular migration is run as user 1 and returns the user value back to whatever it had been at the end of the migration.  It also throws an exception if for some reason the user switch failed.