Google gives you a few invitations (I think 6, but the number may vary) after you've been using it for some time (a week?). IMO, it's quite possible that by the time they will decide to open up their service to the public, everyone will already have an account :).
So far I'm liking it. The service is very fast, and the idea originally-pleasant. Though, it does lack in a few features (A thorough address book system, for one. At the moment it only stores the name and the email of the contact), which I think will be available in due time.
