CAN SPAM + Opt-in

Hey. I read on one website that if you're going to be spamming a non-opt-in list (and of course trying to be CAN SPAM compliant) then you had to take certain measures, such as stating the message was an advertisement on the first line. I've read other places that CAN SPAM prohibits you from mailing to a non-opt-in list in the first place... Does anyone know which for sure is true? Thanks