Hi there, and welcome to the GMAT club forums!
I'm a little curious why you think an MBA might be a good option for you at this point. If you get applications in this fall, you wouldn't be starting until Fall 2011 anyway, and you would then be entering a two year long program which would further delay your entry into medical school. I think pursuing a lab research position, taking some clinical courses in a nursing program, or volunteering in a hospital/clinic might be more beneficial to you by strengthening your application for the MD programs.
To address your question, I think those who pursue an MBA and MD are typically older individuals who have already completed their MD and are interested in moving into the administration side of the health care industry- either in hospital management, insurance, devices, or a few go into consulting (yes, McKinsey hires MDs every year!). If your intention is to be a practicing physician, then I don't think an MBA would be of much assistance. Besides, if you are just out of an undergraduate program, you might struggle to gain admissions to an MBA program with no work experience.
Best of luck in your future career! Let us know if you have any additional questions about an MBA.
"Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.” ~Mark Twain