There's one Canadian in my medical school from BC. I can't really speak as to how common it is nation-wide, though. My instinct is that there are not many, mainly because American medical schools (like Canadian ones) want to invest in students who they feel are most likely to stay in the US to work. If you are sure you want to practice medicine in the US, getting into an American medical school would help you tremendously, as it would put you on equal footing with other Americans when applying for an American residency.