2 years back when I first used gmail.com it was a beta service..Why is it still a beta service?Will google stop its free email service once the beta period is over..Is there any guidelines how long a software can be called a beta version?
The official reason is that Gmail still has problems with scalability and server errors are still frequent.
Here's a very interesting article that'll answer many of your questions:
Gmail was cancelled in some countries due to legal issues and Google have moved many people to googlemail.com /.co.uk domains. All new signups now have to use googlemail.
So Gmail is still available for everyone that originally had an invite, but until they resolve the legal issues, it remains Beta.