I did my undergrad in finance and health care management and worked for a couple of years in biotech private equity before med school. Obviously while in med school, my undergrad was a waste with the exception of understanding ethics and medical jurisprudence which were covered in my corporate ethics, contract law, and health care law classes. But having real world work experience really helps during your clerkships years.
While MD programs are all pretty much the same with respect to their course offerings, MBA programs are very diverse. SO DO YOUR HOMEWORK. Some MD/MBA programs have health care management & policy majors while some have only health care *classes*. Some don't even have any health care classes.
All will have your basics: finance, accounting, management, marketing, information systems, business law... which should be sufficient in teaching you principles and how to think about things. In B school though you'll sample case studies from different industries and so unless they have a health care department, you may or may not get as good of exposure to health care and the insurance industry.
So depending on the program and what classes you pick, yes, MD/MBA could be really corporate focused: health care investment banking, venture capital/private equity, hospital management consulting, health care information technology consulting, employee benefits consulting, managed care consulting, pharmaceutical corporate management.
Or you can pick classes that teach you small business/entrepeneur management and marketing if your interested in running your own clinic/hospital/start up company. These classes require that you write a business plan, present it to your faculty as if you were presenting it to a bunch of bankers.
For some people, they get their MD/MBA get a job in Corporate America and never go back to medicine because the pay is SOOOO much better, you get to travel, the hours are SOOO much better, and you don't have patients whining. Others get their MD/MBA and start their own biotech/medical device company, while others become regular doctors and/or administrators. There's a lot of flexibility with the MBA portion, but realize that they only teach you the "theory"-side of business. The majority of your business understanding will come from your pre-/post-MBA work experience.
Remember that if you don't get into a MD/MBA program, you can always get the MBA later (maybe at an even better program than the one offered through the joint degree). The reputation behind the MBA is really important (regardless of how great an education you got... again, different from med school). But if you were to get your MBA later in life, you should only do it if you're looking to dramatically improve your life situation (ie become CEO, change careers). It's not worth getting an MBA (2 years of debt + 2 years of loss wages) if it doesn't pay you off. There are a lot of people out in the business world (with bachelor's degree, some in business, others in "would you like fries with that"-degree) who think and MBA is a waste of time and money especially when you can work your way up. In certain situations, these guys have a good point.
Oh yeah, there're also MPH programs that offer health care management degrees, but tend to be weak on the numbers-side, so depending on what you want, this may or may not matter.