"Pro" camera is any camera that allows a professional to get the job done.
The term "Professional" to identify products is an oxymoronic marketing term. The only other marketing term even more idiotic is "Prosumer". Kinda like the "Certified Preowned vehicle".
I think the manufacture decides what level a camera is at.
Like
Pro- D3s D3 D3x
Semi Pro- D700
Prosumer - D300s
Consumer- D90
Entry- D3000 D5000
Not to sure about canon though. lol
Nikon/canon marketing use those terms to classify the camera bodies they sell; however nothing stops a "Professional photographer" from using a D90 for pro work.
Juza Nature Photography
350D - entry level and outdate camera body by todays standards and yet pro grade work being produced.
It's really only forum people that really get in a fit about "pro" and the word should be banned from forums

A professional photographer is just a person who makes money by using a camera as the main part of their profession. Thus any tools/gear they use is by default professional grade - but that photographers grading. Different situations, requirements and output demands will determin what gear the professional needs in order to achive the end result.
Further in all these aspects there will be a range of products that can do the job from low to high budget. Again only forum surfers tend to get in a fit if the pro is not using the most highly priced options for their work. Though often there are oddities - such as many saying a 5DM2 should be the minimum for a wedding photographer - but they dare not suggest that the best tool is infact a Hassy (mostly because theperson giving the view dosn't own a hassy

)
In the end you have to decide for yourself based on your needs, what kind of output you are expected to produce, the level of quality you feel is nessessary and also the budget that you have to work with. For example a rebel camera body can be used (and has been) to shot a wedding and the results can be very good - however offer that person a 7D or even a 5DM2 and the advantages of those bodies would allow far more technically superior shots as well as expand the conditions that the photographer is able to shoot in.
Finally never get into the mindset that 1.6 crop camera bodies are "lesser". They are a different crop factor to fullframe and the only real area they are "lesser" in is that they don't do super high ISOs as well as the fullframe sensors. There are many pros working with crop sensor camera bodies (mostly 1DM2/3/4 or 7D) such as sports, wildlife, reporter etc....