I spent two years and three months in Bolivia as a Peace Corps volunteer in Nutritional Agriculture and loved it!
In someways the Peace Corps is just PR for the US governement so that people from other countries will like us and continue to buy Coca-Cola and let us continue with the lopsided dichotomy of US/Developing world economics. But I enjoyed it and would recommend it to anyone.
Postitives: learned a great deal about culture and human nature, became almost fluent in Spanish, traveled all over South America. Once you are in medical school your traveling days are limited.
Most difficult aspects (but in the end worthwhile): emotionally challenging at times, culture shock of returning to the US.
When I interviewed at ATSU-Mesa one of the Professors I was eating lunch with recognized my name and told me that when she saw the Peace Corps on my resume she put a big star on it. Now I'm part of the class of 2011!
You should also check out Americorps. Its only a year long and you can work at community health clinics here in the U.S. The people I know who did Americorps got into schools like UCSF and John Hopkins.