I have an account with writely.com which wrote and introduced one of the first web-based document editors. It was bought out by Google and they immediately suspended new accounts until they re-branded the product as part of Google Docs. I think the same will happen here. It'll happen, but might take up to 6 months or so to accomplish (purely my personal guess, here).
Either way, there are people who have invites to grandcentral.com as we speak which will allow you to sign up and that's what this thread was started for/about. Thanks for your posts, but they're really not providing and new insight. 🙂