I think it's open to the public now. I got an invite from gmail not too long ago saying I could have an account now. I put myself on their mailing list and they finally emailed me. I already had an account by then, though because someone invited me. I have 50 invites, too. :-)
I think the invite thing is good because if you invite evil folks and they do bad, then its your fault and you're linked to the badness so you try to only invite good people you know.
