As far as I know you have to go six years of residency in OMFS or apply to med school afterwards, or apply to med school after dental school and have a good reason as to why you got your DDS and want to spend another 250k to get an MD (and not really utilize it). However, my girlfriends father, a graduate of UPenn dental was offered the chance to spend 6 additional months after graduation to receive his MD. He opted not to do it as he did not see the added benefit and really was ready to finally enter the profession after four long years of school.
IMHO I think dental schools such as UPENN, Harvard, Columbia, UConn, and many others are incorporating so much overall medical knowledge in the curriculum (especially those that combine med and dents in years 1 and 2) that there is virtually no added benefit to getting an MD, as you will already have much of the knowledge of that of a physician. I do not mean to offend you in any way or question your intelligence, but I think if you truly want some honest advice on this matter you must first be willing to tell us why you in fact want the M.D. and how you think it will contribute to your success as a dentist. Is it that you want to be a dentist or a physician? You applied to dental school, so why are you disatisfied with receiving a DDS? Patients typically go to an MD when they have a systemic ailment that needs treatment, not to an MD/DMD who pratices dentistry. You will rarely use the additional knowledge acquired by getting an MD as a dentist, and that sure is a lot of extra work, money, and time to get an MD simply for prestige.
I think the problem is that up until this point many of us have viewed success by how many things we can put on our resume, but now is the time to mature in our way of thinking and realize that we are going to become a doctor and that is truly the peak of the success we have all been striving for, it doesn't matter anymore how many things we do or how many degrees we get. We don't have to impress an adcom anymore...ever. Now we get to hone in on our skills and become a service to our communtiy.