Forbidden Questions: Drop MD portion? Payback?

These are the kind of questions you can't ask at an interview, or they'll boot you right out the door. They are important, though, so I'll ask them here. I think I mentioned one once but didn't get much of an answer.

If you are accepted to an MD/PhD program, then change your mind, can you accept the MD seat (w/o any funding, of course) but decline the PhD position? Are there differences between schools?

Also, I have heard that the NIH funded MSTP programs do not require payback of either stipend or tuition from MD/PhD droppouts. Is this true? What about non-NIH programs? Which do/don't require payback, and for tuition or stipend or both?

Thanks!