Probably the same marketing technique as Gmail, Google realise that everyone wants to be part of the exclusive club especially webz0rs like us. So by making it invite only you'll soon see fellow webz0rs like us selling invites on ebay and forums despite the fact it'll be free for public use soon
Replace webz0rs with insult or similar
Awesome annimation as well. Simple, to the point, Non distracting and above all - informative.