It depends on the schools.
If the mission statement of the school says it wants to create physicians that do research, then it's a plus. However, at a school that receives little research funding it is a negative because they could see this as a reason you would attend a different school.
If you want to do research in medical school then make sure you apply to schools that have research as a mandatory part for graduation, mentions it in their curriculum, receives a ton of funding for it or has placed it in their mission statement.
To be perfectly honest, I would say that it's extremely unlikely that saying you wish to do research will harm your application. I can't say that it will help at every single school, but I will say that I come from a private school that has very little money for research and my interviewers were impressed that I had a publication from undergraduate and were eager to tell me about research opportunities inside the school. They also were aware of numerous research opportunities outside of the school.
A great way to frame your interest in research, especially for schools that don't emphasize it, is to mention one or more of the following:
1. Understanding research can help you better serve your patients; drug companies can be misleading in the way they market drugs, and being able to read papers about the particular drugs can give you valuable information about whether or not a particular drug is appropriate for your patient, especially if it's one that your practice does not regularly prescribe.
2. Research studies are sometimes the only way that underserved patients can receive care; many psych studies, for example, will offer free counseling services in addition to whatever drug they're testing to treat a condition. A poor patient with depression who can't afford counseling but needs it desperately may find this kind of study a lifesaver.
3. Research helps to develop new drugs and therapies (not to mention knowledge of basic structure and function, for all of you basic scientists out there) that will eventually help us treat disease better.
As long as you frame it right, it's extremely unlikely that mentioning an interest in research will hurt your application, and if you spin it right, it can be a tremendous asset at almost any school.