I have both an undergrad and a Master's in biomedical engineering. I don't think a M.S. in BME would hurt your chances of getting into med school, personally. If you are doing so purely as a means of getting into med school, then I wouldn't recommend it, as there are easier ways to boost your app. If you're interested in it, then go for it. I found BME graduate school to be much easier than undergrad, and because this is known, adcoms expect you to have a very high grad school GPA. Besides, if you do a M.S., you'll likely have a research position that will pay your way through school, and you'll add to your network of people who can help you get where you want to be.
If you want to get a BME job, however, don't bother; I think medical companies are more likely to hire EEs for medical device positions, or MechEs for biomechanics positions.
In response to Sacrament's post, I think the experience in BME programs will vary widely, with some programs a lot more practical than others.
Regarding Fermata's post: If you don't didn't learn *any* circuit theory, you might have a hard time jumping into a BME M.S. program. One of the first labs in grad school involved designing and building a basic EKG and respiratory flow monitor, and the people who came in from biochem, or bio, or medical backgrounds had a really difficult time. Otherwise, it's not a bad idea. 🙂