Coming from a school - Tulane - with a very famous MD/MPH (the first and most prolific ... unfortunately not the most challenging or prestigious), I'll tell you that if you got into the MD program, you have the choice of whether you want to do the MPH or not. Don't even worry about it now, don't mention on your app if you don't want to. The earliest you can start is second semester of first year. And they recruit hard, because they can roll you for another $10,000. So, just wait till you get the MD acceptance, and then talk to people. I'm already accepted into the MPH, but I'm not even sure I'm doing it anymore. It's kind of watered down, you take much less courses (some say that the med school courses would overlap, I don't know about that...).
People have mixed feelings about Tulane's MD/MPH program, and after I get more information, I may want to hold off and instead getting a watered down "slash-MPH", I may want to try to get a full-frontal "comma-MPH".
I think the classes look very fascinating, and I'm definitely going to take a bunch of them, but I don't know if I want to give up my final summer (after 1st year med school) taking classes here. Someone has to enlighten on me whether an MPH would put me in a stronger position, or doing a research would...
Good luck,
Simul
Tulane Med '05