Okay Bostondiner here goes: I found this on Wikipedia which explains the titles of surgeons, doctors, dentists and some of the differences between UK and US titles.
In English-speaking countries, the title doctor is strongly associated with the medical profession. Most medical practitioners use the title professionally and socially.
In the United Kingdom and many Commonwealth countries, those training for the medical profession complete either a 4-year graduate entry or an accelerated 5-6 year course leading to the degree of Bachelor of Medicine and Bachelor of Surgery (MBBS, MBChB, or other similar abbreviation);the higher postgraduate degree of Doctor of Medicine (MD) is reserved for those who can prove a particular distinction on the field, usually through a body of published work or the submission of a dissertation. In guidance issued by Who's Who it is noted that in the context of the United Kingdom, "not all qualified medical [practitioners] hold the [MD] degree" but that "those ... who have not taken [it] are addressed as if they had."
A & C Black also note that British surgeons - a designation reserved for those who have obtained membership of the Royal College of Surgeons - are addressed as Mr, Mrs or Miss rather than Dr. This custom has been commented on in the British Medical Journal and may stem from the historical origins of the profession.
For many years the UK's General Dental Council (GDC) regarded the use of the title doctor by dentists as a disciplinary offence; however on November 14, 1995 the GDC ruled that dentists could use the title doctor thenceforth provided that they did not do so to imply that they held qualifications that they did not possess.
Speaking in the House of Commons of the United Kingdom on January 19, 1996, health minister Gerald Malone noted that the title doctor had never been restricted to either medical practitioners or those with doctoral degrees in the United Kingdom, commenting that the word was defined by common usage but that the titles "physician, doctor of medicine, licentiate in medicine and surgery, bachelor of medicine, surgeon, general practitioner and apothecary" did have special protection in law.
In the United States and other countries, the basic medical qualification is the M.D., usually completed as a second entry degree following a Bachelor of Science (B.S.) degree. The United States M.D. is the equivalent of the British MBBS, MB, BMed, MBChB, etc qualifications. Other health-related disciplines such as podiatry and dentistry use a very similar educational framework, though a podiatrist is often referred to as a "podiatric physician/surgeon" and a dentist a "dental surgeon" or "dental or oral physician".
In the Unites States only, the D.O. degree is an equivalent degree to M.D. with a different history.