Something I noticed about "things", when a person signs up for the newsletter, it is showing them on the dashboard and under Sales>customers as new customers. Then the admin has the choice to approve them or not.
Well, if a newsletter subscriber is approved as a customer by admin before they actually register, then they can't create a new account because A-cart returns, this email address is already in use, or something to that effect..
A suggestion for future additions is to disallow the admin from turning the customer status on when a person only signs up for the newsletter without creating an account. If the status is off, a customer that has signed up for the newsletter and then chooses to become a customer can, otherwise, they can't without having an alternative email address. Follow?