I agree that there are huge needs here in the United States, but they simply do not compare in the least to the developing countries. I have lived and worked in Ethiopia and spent a month in the rural Sierra Madre mountains of northern Mexico and those experiences are something else.
In the US we are concerned about an infant mortality rate of 7/1000 births and a low birth weight % above 5% and mothers getting prenatal care in the 1st trimester, but in these countries, the infant mortality rate is more like 10% and in some countries, it approaches 50%.
I also definitely agree with the suggestion about getting an MPH. This will broaden your view of health and give you some skills that are more useful abroad. As you might be able to guess, the problems in these developing countries are more infrastructural and one doc seeing as many patients as possible doesn't really begin to make much of an impact. Couple your clinical skills with some population-based knowledge and ability and you can make a more profound impact. I have my MPH and am trying to get into Med School now.