1. Consistantly Winning
2. Consistantly able to raise the stakes at which you win at
I would think you could be a "pro" @ $.01/$.02 tables, but that's probably not what you are calling a "pro"(?). So consistantly able to win and raise your stakes and continue winning would probably be what would make someone a pro.
I wouldn't think just having poker as your source of income would make you a pro. What if you have a 9to5, but then go to Vegas every weekend and are consistantly winning, and are able to raise the stakes every year or so. Or, you have a 9to5 but you go to 3 or 4 high stakes tournies each year and always place in the $$?
Also, once a pro, always a pro?