It depends on the school. When going through the MSAR I saw some schools specifically say that people with graduate degrees were considered the same as those without (ie you have no advantage/disadvantage with a graduate degree). But there are definitely some schools that are very interested in taking people with PhD's for the research aspect of it. Just check the schools and make sure they are for research. I met a resident that applied to a family practice school (she had a PhD already), and was instantly rejected (and it was her in state school). She ended up getting into Mayo. So be careful. If you do want to apply to non-research based schools, I'd make sure your PS states that clearly. If not you may be looking at quick rejections, because they assume you want to do research. But overall, you should get in somewhere. I think a graduate degree in a good thing on your application. 🙂