It's tough because there really is a lot of debate in psychology about what the definition of addiction is. I guess my understanding is based on something like incentive sensitization theory, which suggests that everything we do is a matter of incentive (and addiction is becoming gradually desensitized to the effect, while more sensitive to the trigger/desire). In that sense everything is a potential addiction, including stuff like food, relationships, gambling, drugs, etc. When you're not addicted there's a healthy balance between all these facets of life but addiction is where the balance gradually shifts so one aspect takes on a disproportionate amount of focus.
In that sense the only difference between an addict and a functional addict is that the functional addict maintains their attachments to other facets of life. I guess the further thought is that with decreased incentive towards those other things, it is likely the person will eventually slip into non-functional addiction.
After writing all that honestly I'm not sure if I'd still call myself an addict because pot doesn't seem to eclipse my other desires much at all.