Just as FoughtFyr and our favorite resident Mom, I too pursued a two-year MPH degree in Health Education, followed by two years of fellowship with the government prior to medical school.
I won't get on my soapbox here, because I think that the issue has been beaten like scrambled eggs, but the public health/MPH degree in and of itself is sufficient depending on the kind of work that you want to do. I have plenty of friends who are working with MPHs, as well as a number who are pursuing research careers and have completed (or are close to completing) their PhD/DrPH degrees. You do not need a PhD to ultimately survive in public health, but if public health is what you want to do, the MPH is pretty much necessary (although there are folks in public health without the MPH, esp those working for not-for-profit organizations). The MPH in conjunction with a professional degree such as the MD is valuable, but not so valuable that you should get the degree if you're only using it to get into medical school.
http://forums.studentdoctor.net/showthread.php?t=134791&highlight=heal&teach
One more point that I will make is that I have no regrets about going the route that I did, but in terms of saving money and time, I currently advise students that it is more worthwhile for you to do the MPH during medical school (especially IF this is ultimately what you want to do). However, if you are pretty unsure about the decision between public health and medicine (two distinct areas that are slowly trying to reunite on many fronts) and truly interested in public health, then get the MPH and see where things take you.
Best,
H&T