I should add that the people that are in the program now have started the summer before the school year starts. So the program is actually a summer longer than six years. The schools that offer DMD/PhD programs can justify the expedited degrees because, depending on the major, many of the dental school courses (biochem, anat/phys, biomaterials, etc) overlap with the PhD course requirements. Also, students can perform lab work and write up their thesis during free time from dental school, summers, and, obviously, the extra two years. It's not for everyone, but if you have the research background and academic ability to enter the program, the program treats you quite well. I've traveled across the states (and Germany) to present and do research, had my dental school and all my living expenses paid for, and been exposed to some top-notch research and researchers.
Sorry to go on about it, but when I saw that over 400 people have read this thread, I thought that I would add a little more. I know this might be obvious information, but when I was browsing through this website as a pre-dent, I ran into a lot of bad information and read many things that I later found out to be untrue. If you are sincerely interested (or just thinking about getting a DMD/PhD) I would encourage you to contact any of the schools that have the program (I know Baylor, UConn, Maryland, San Antonio, and UAB have them for sure..there are definitely others too). I can't speak for any other school, but I know you can send an email to Dr. MacDougall at UAB, she's extremely nice and would be very helpful.
Nate