I am a student in the 2007 class who is finishing a master's in education this week. There are ten of us in our class completing a double degree and only about four the year below us. Essentially, they have some programs (education/bioethics) that are set up to accept dental students. However, if you are interested in earning something else, you simply propose the plan to the dental school academic affairs office, get accepted to the program, and then they'll decide on a case-by-case basis. I know one student is earning their MBA here at Penn but had to take off a year of dental school between years two and three to complete it (others, like education, can be done concurrently within the four year time span). Essentially, though, if you are really interested in earning a dual degree of some sort, they'll let you. Let me know if you have any other questions.