Hello Rob,
I'm not sure exactly what you are asking, but I can try my best to explain where portal registrations are kept. The portal login is contained in whatever table you chose to as the table to base the registrations on. We can take a look at the default Contacts_Portal. It requires the Contacts table to have three fields for the User name, password and email. It users PortalUserName, PortalPassword and the Email field respectively. If a customer registers, you can see values in these fields on the table. Any blank values means the customer has not registered for the portal.
There are no tables or audit trails that give you when the user logged in or when they registered for a portal. There account is really the record where the portal registration information is placed.
Greg