Before you get too involved in the MD vs MD/PhD debate, you need to consider a couple of things:
1) Do you really see yourself with a research-predominant career? MD/PhD programs are really designed for those who plan to run their own basic science labs and split their time about 80:20 between research and clinical medicine. If your dream is a bit different (working primarily as a clinician and publishing clinical research in between), then you would probably be wasting your time with a PhD. PhD programs are phenomenal if you need to specialized basic science experience, but they aren't the right choice for everyone.
2) Do you have a realistic shot at admissions? If you want to gain admission to an MD/PhD program, your research record needs to be excellent. This usually means at least two years of responsibility-bearing lab experience. Most students have carried out their own projects, written up, and presented their own data. If you haven't done that then you probably need more experience under your belt before you apply.
Also, although research background is most important, high MCAT scores are expected of MD/PhD graduates. A safe number is something above a 35. Lower number will cause you some heartache.
At the completion of my first year there were 3 students who transferred to the MD/PhD program, making the total 7 for our year.
This is not uncommon among schools but it really depends on a school's funding status. If you decide to apply MD-only, you can always email the MD/PhD program directors to see if internal applications are common for the school's MD/PhD program.
Sort of the opposite of the above post but... Another loophole that I've heard of people doing (that sort of irks me) is that people can drop out of MD/PhD programs and not be required to pay back the free tuition and/or stipend.
Schools try really, really hard to keep these people out of MD/PhD programs, and I think that those who do creep through with no intention of earning a PhD are somewhat of a rarity.
As for paying money back though, there are a few rules. If you are a student in a program with federal support (MSTP), you cannot be required to pay back federal money (the government would consider this an undue pressure for someone to stay in the program). If schools use private money to pay tuition and stipends, however, they make their own rules about repayment.
just be aware that if you apply to an MD and MD/PhD program at a school, and if you are not accepted for an MD/PhD interview, the school will then look at your application to see if you should be invited for an MD interview.
Many, but not all schools automatically consider MD/PhD applicants for the MD program. It is not uncommon for secondaries to have a check box asking: "If you are not accepted to the MD/PhD program, would you like to be considered for MD admission?". Also note that some schools (usually top schools) allow no cross consideration at all.
PS - there is also an MD/PhD forum that you can visit for more information.