On an old email address I signed up to be informed of gmail update way back when it first opened. Today I received an email from google offering me an account (I already have a few from a personal invite).
So now google is opening it up to people who signed up to stay informed. The next logical step is to open it up for everyone, it's going to happen sooner rather than later.
This just coming after they allow 50 invites, it's coming soon. Wait until MS or Yahoo do something big, the next day google releases gmail.