Does Carnival automatically assign everyone who has either registered at their website or cruised with them before to a PVP?
Not to my knowledge. I have been registered on the site since they started registering, and haven't had one call me. Maybe certain applications on the site could trigger it. I know they want you to register to get the new brochures I posted about.
It would be funny if one did call me, I would play stump the PVP.