Russ do you have any figures? I'm curious now as to how much they earn. Also, I didn't mean cardiology as 'on average', but when you include cardiology and radiology private practices, they definitely lift the average up.
I think what everyoneis saying is that on average, dentists earn more than doctors, but obviously if you are a neurosurgeon with private practice then you would exceed the salary of a dentist, if you had enough patients.
Dentists have one of the highest undergraduate first year out salaries (whatever you call it, there's a more eloquent way of saying it) and then it kind of plateaus, but read below for how a dentist can out-earn some established doctors. Unfortunately, I don't think this will last for long and I think the career of dentistry won't be as financially rewarding in the future.
You have to consider that currently, medical practice has many ties with government policies like Medicare.. which affects how much you earn. Right now, for a health service, dentistry is pretty private and many dentists have control over how much they want to charge you. And yes, a dentist can out earn even a specialist like a cardiologist if they are an unethical dentist that sees patients in and out in ten minutes or insists on filling teeth which could be "re-strengthened" naturally. My friend reports that his father's earning potential is half a mil each year.