I've heard conflicting things from different people. HMS/HBS said that a lot of people wait until they get in to the med school to apply, partially because HBS is so selective. But someone else said in private that you need to have someone vouch for you to be able to get in.
Chicago/Booth, Yale/Yale SOM, Stanford etc. all say that you apply after getting into the med school. Silverman (Yale Med) in particular says that this works better for the business schools because they get the diversity of having the healthcare perspective while knowing that they're still recruiting highly competitive candidates.