If you need to import content that's unpublished, but scheduled for future publication, you can do so by setting a node's 'publish_on' property in your Migration class's prepare() method implementation. For example:

  // In construct(), you have a row that has a published status of 0:
  public function __construct() {
    $this->addFieldMapping('status', 'status');
  }

  // In prepare(), you can set the timestamp for the time the node should
  // be published (requires scheduler.module).
  public function prepare($node, stdClass $row) {
    if ($node->status == 0 && $row->status == 0 && $row->publish_time >= time()) {
      $node->publish_on = $row->publish_time;
    }
  }