Servicing an underserved population (at least as far as schools are concerned) means your interested in primary care. For my state school, that includes emergency medicine, pediatrics, internal medicine, and family medicine. The National Health Service Coorps also includes psychiatry and some states include Obstetrics/Gynecology. I believe an underserved area is devided by county. I don't believe that just because you work in an underserved area means you have to be completely out in the boondocks. For instance, in Kansas (my state school) all but four counties are considered underserved. One can even live and serve in Wyandotte county (part of Kansas City) and be serving an underserved area.
I plan on serving in rural primary care and am sure that was one of the reasons I got accepted this cycle. I also grew up on a farm, so I'm sure the adcom believed me. Living in a rural area can be a fantastic thing... and living in a rural area doesn't mean living far from civilization (as I already stated) It took me just as long to get to a Walmart in my hometown (11 miles away) as it does for me to get to a Walmart now. (live in Kansas City).
