This question arises in the world o' sport, does it not? (think that's been hinted at here) - After all, there are professional athletes and amateur athletes. Methinks the line has been blurred by the fact that so many "amateur" athletes receive monetary recompense of one sort or another (grants, scholarships, sponsorships) for their training....

I was musing on this with regard to artists' models. Some of us are "professionals" - we are paid to do the work and we do it well - and we won't work unless we are paid. Some are clearly "amateurs" in that they offer to work "for prints" or "for experience." REAL models (us professionals!) avoid such types like the plague. I made the mistake of working with one once and he was quite plainly only working the gig for titillation. (And he was a lousy model.)

For me, the term "professional" includes, among its many implications, a high degree of respect for the work being done and a knowledge of that work's worth. Applying these criteria to modelling, then, I would say that even some models who are paid to do the work, are NOT professional about it. "Professional" also incorporates a degree of dignity, does it not? - not stuffiness, but dignity....