How to slow down migrate? Cloud Files can't keep up
In your migration scripts you can use either prepare() or complete() (http://drupal.org/node/1132582) and add in sleep(). It's a hacky solution but it should give you enough time between processing each row for Rackspace to catch up.

public function prepare($node, stdClass $row) {
  sleep(30); //wait for 30 seconds
}