I have sold my D90 which was my limited use 3rd camera for professional use. I plan on getting a D7000 to replace this need. Based on what I have seen so far, I think the D7000 can be used in several professional capacities.
I think in the end, what constitutes professional photography is what buyers are willing to purchase. If somebody with no training hangs up their shingle and declares themselves a pro and people are willing to pay them money, then in my book they have met the minimum requirements. But just because someone is willing to pay you does not make you a "good photographer" or a great one.
I shoot mostly weddings and the prices I charge reflect my capabilities and training based and on what my market will pay for these skills. There are many wedding photographers who charge more and deserve it. There are countless others who charge less. My work holds up well against them and I have no problems charging what I feel that I deserve, especially when I see some other "pro" photographers work.
There are formalized degrees that one can obtain in photography. And there are also certifications. Just as in in many other professions, a degree does not guarantee professionalism. And nor does the lack of a degree indicate the lack of expertise. I have seen excellent wedding photography from shooters without any degrees. In this particular line of photography, customer referrals and personal tastes account for most of the buying decisions. If your work looks good, people will hire you regardless of any certifications. In fact, I cannot remember one prospective wedding client who ever inquired about my qualifications in many years. They do, from time to time, inquire about what equipment that you use. And this is one of the biggest drawbacks from using a camera like a D7000 for this work is the perception by some clients that if you are using a camera that they could afford, perhaps they could get a friend who also owns one to shoot their event just as well. Since I have a D3 and professional lenses, I have never (knowingly) lost a client due to the camera that I use.