From what I understand, it's not PUBLIC public. I think they have extended thier invitations to google members/google users. Or something like that. Read it on a news article about it. They are still in beta though.
Ok just checked it again. They are randomly offering users on thier main google search page, to open a gmail account. So users who use thier google.com page to search, might get an invitation to open a gmail account.
But it's still in beta. It looks like they are preparing to go public any day now.