I am about to finish medical school and I have a BS in nursing. Each school has their own requirements. They are mostly the same, and some of the classes you will take as a nursing student will could to your requirements for med school. There are also requirements that are not required for most nursing degrees (organic chemistry, physics, etc) and you will need to take these courses with your nursing courses. You should contact the admissions office of the medical schools via email, and look at their websites to find out what classes you need. As long as you meet these requirements, it doesn't matter what your degree is . . . I have class mates who have degrees in a number of fields. With a nursing degree, you will gain clinical experience, communication skills, and some knowledge of the health care system which will set you apart from all of the biology and chemistry majors that look very similar on paper applications. Do whatever seems right for you, just make sure you take the courses you need, put some time into community service (you will have chances to do this in nursing school) and join a club or two. Talk to a pre-med advisor at your school if you get a chance. My nursing instructors were very supportive as well. Most important, contact with the admissions person at the med schools directly to answer your questions.
Good luck.

You don't have to major in a science / medical field to count as pre-med. To be pre-med, you have to complete the requirements for medical school, which includes 1 year of general chemistry, 1 year of organic chemistry, 1 year of physics, 1 year of biology. If you've completed those requirements in the course of your BS in Nursing, then you probably are okay.