Great question, I am a dentist and took a forensic's class in dental school. I can't say I know the specifics, but I know it takes a lifetime to get certified in my state. I remember my Course instuctor saying it took (i'm guessing) 15 cases to complete in order to be certified as a forensic dentist but he only averaged 1 to 2 cases per year. I have had one case in 4 years that I had to send radiographs to a coroner. Most often you would be dealing with bite marks and patient identification. As you can tell, there aren't enough cases to keep you busy full time. You must have a strong stomach as patient identification is typically used when bodied are decayed beyond all recognition (plane crashes, floaters, burns).
Also I agree with the above person. Most dentists do it as a hobby.

Update...I found the site for the American board of Forensic odontology they have a requirements posted
check it out at http://www.abfo.org/