Funnily enough I was told a different story from that at the Wikipedia article. They told me that by the XVII century there weren't really surgeons (mostly cause there weren't anesthesiologists either) so anything which had to do with draining festers and the like was performed by dentists, which were considered an inferior class than "true doctors". Dentists of course weren't doctors but misters. As medicine and surgery evolved and became complementary parts of health care, which you needed a medical degree to practice, the tradition kept the different addressing titles, and always gave some preponderance to doctors over misters.
I guess someone made that story up to prove a point about British being social class minded people, or whatever. Maybe one thing is what surgeons want to believe and other what people think hehehehe.
Take care Dan, you'll be enjoying your IEMs in no time