I've considered a career in practicing medicine and at some point in time teaching at a medical school and/or affiliated teaching hospital. What are the paths that one usually takes to get to the academia side of medicine? Do you have to obtain a PhD on top of your MD or is simply kicking ass along with knowing people who know people the way it works?
All of the above.
For the most part, here at University of California, as a professor (full or associate), you are expected to be:
(1) National and/or international leader in your field of study
(2) Actively doing research and coming up with new things.
Which pretty much comes down to publications. So as the saying goes "publish or perish". Having a PhD is certainly not required, but many, if not all med school professors have large amounts of research. They gained this through fellowships, internships, masters or PhD programs.
Doing a 1-2 year post-doc is not required, but rarely do people get into a faculty position right after a PhD program. MDs may be a tad different since they always teach, but to be appointed as a tenure-tracked professor, then you should have a good foundation in research. This makes the university comfortable with you making money for them. Thats what it really comes down to.
Might also help to see if you have a good track record at getting funding (e.g., NSF, NIH, etc). Experience is key in his arena. The more publications you have, the more talks you do, and so forth, the more people will know about you. This would be the driving force which gets you into a good position in acadamia. Good times.