I think that you can generally be considered for both MD and MD-PhD at most schools. However, the way in which the dual consideration occurs differs from school to school. At some schools, you are first considered for MD-PhD and then funneled into the MD pool if you are rejected from MD-PhD. (At schools with rolling admissions, this may negatively impact your chance of an MD acceptance.) Other schools require you to be accepted to the MD program before the MD-PhD committee looks at your application.