Most, if not all, MD schools will want you to have a finished BS/BA by the time you enroll. However, considering that if you were to take the MCAT in April and apply this year, you’re still a year and half away from starting MD school, since you’d be applying for fall 2005 –plenty of time to finish your degree. DO schools might let you slide without completing your BS, I don’t know. Call the DO schools you’re interested in and ask them. You can still take Physics, and the MCAT and apply this year to both MD and DO schools; an incomplete BSN isn’t going to prevent you from getting interviews, though any acceptances might be conditional on you completing your degree.
Also, if you do decide to go to DO school, keep in mind you may be less competitive in the residency match process for many specialties. Outside of the primary care specialties (which are generally less competitive) — family practice, internal medicine, pediatrics –it is difficult for DOs to match ahead of MD students. These are specialties such as Dermatology, Surgical subspecialties (ENT, Plastics, Ortho.), Radiology, Opthomology. If you are positive that you are not interested in the more competitive specialties, then you lose nothing by going to DO school.