Wow, this thread hits home. I took all my medical school prereqs in college (graduated '97) but then I let some relatives in the medical profession talk me out of med school, because I was young and impressionable. Because I am a little neurotic, I had to find something else which was "impressive," so I made the middle class mistake of going to law school. I received my JD in 2000. I thought law school was fun. I made great freinds and had tons of free time. I have basically despised being a lawyer ever since law school graduation, even though I have one of the "cushy" (anything but) jobs at an AmLaw 100 law firm making a nice salary. The problem is, if you do not want to go to your job in the morning, it does not matter how much you get paid. Anyway, for the past few years I have been working to get myself into medical school. I have had a few interviews and am hopeful it will work out for me to be in the class of 2011.
As for the subjects of this thread, there were two MDs in my law school class. One dropped out while the other was a Supreme Court Clerk. Anectdotally, it seems every medical student I talk to knows 1 or 2 lawyers in medical school. So without any scientific basis, it seems that more lawyers become doctors than vice versa. Personally I cannot imagine going to medical school if you did not intend on working in the medical field as some have suggested.
No one has directly indicated that being a lawyer is detrimental to my application. However, I have still not been invited for interviews at a couple of schools where my objective numbers are significantly higher than the schools admitted students' averages, so I could believe that the JD is not exactly a gold star in the AMCAS application - especially when you have been practicing law 50-60 hours a week instead of doing summer internships in African clinics or assisting the world class cancer researcher at your undergrad school.