They're sending them out in the order they received the orders. Technically, everyone that ordered before you did should be online before you. This is done to be fair in terms of getting a chance to secure your username. In practice, it seems to just be making everyone angry that it is taking a long time.
Still waiting for my code with you. It'll get to us eventually!
@JC Sending out hundreds/thousands of emails at the same time is actually proven to be quite difficult. It's not quite the same as just sending one email. They need to make sure the emails are actually sent out and not get stuck somewhere in the interwebz.
Of course this is just one of the reasons I'm guesstimating. Maybe they're trying to spread out the emails so not everyone's logging in at the same time and crashing the server.At this point let's just hope for the best and hopefully you'll get your code soon.