There are many different definitions from one who simply clocks in and out to collect a check to one who works in a deep way because of love of craft.
It seems like if we want to arrive at a commonly agreed upon base line for defining what a professional barista is then we should set the standard in a place that reflects all the areas a barista should have a grasp on in order to be considered a professional.
The BGA list of competencies is a great place to start. What is needed to add to the standards set forth in the base line is true, trusted, and traceable apprenticeship within the professional environment that exemplifies as a reality in the cafe the ideals the candidate hopes to embody as an individual and is generally agreed upon by the greater industry are ideals that reflect well on them.
If you pass 1,000 tests (either practical, or written) I would not call you a professional as readily as if you had gone through the gauntlet of quality cafe experience over years and had no tests but the recommendations of those who witness BOTH your technical ability AND your ability to be consistently capable in a wide array of glamorous and unglamorous tasks that make up a baristas daily life.
Our definition as an industry can either be codified to allow easy access to the rank or "Professional" through academic testing or it can be made to vet an individual through both test AND testimony much like a chef who graduate from the CIA will still need to earn rank in a kitchen before attaining higher professional levels. More often than not it seems those who hire based of academic credentials reap a certain immaturity and (warning: pun ahead), lack of seasoning. The later form of vetting people before granting them a professional status I think, in the long run, bodes better for an industry where too many claim pro status on paper (and by that I mean accomplishments that are outside the cafe such as certificates from schools, comps, and organizations) and don't even meet status quo in the cafe.
So, to the point, a professional barista should be one who has years of quality bar work in quality driven cafes under good trainers. They should not only know the BGA's core competencies but also possess the proven ability to self govern, handle conflict, lead/train, and perform basic administrative and cafe maintenance functions, as well as be given to and excel in hospitality.
These are just some general ideas that come to mind when this subject is discussed.
Great topic...thanks, Cole.