I believe that an ms degree can help you get accepted. I am currently finishing my ms in anatomy at Case Western Reserve. This degree is basically the first two years of medical school but with more concentration on anatomy, histology, etc. I believe that the degree was definately a an improvement to my application and I believe it helped me to get accepted as well as prepare me for med school.
I think you should concentrate more on what you get out of the MS program than the degree itself. I don't think adcoms would care much if they simply saw a "masters degree" on your transcript. However, if as mentioned above, you take tough, challenging courses - and do well on them - than that'll open an eye or two. In addition, these courses, like anatomy especially, would probably help you in med school classes.
Also, if you do a thesis and possibly publish, then that alone can give you great research training and carry weight, as a publication, later on in your career.
It's what you do for the masters degree that counts more than the degree itself.
Georgetown has one if you're the heir to a small country or a multimillionaire. It's very pricey but supposedly one of the best in helping those who didn't get accepted to med school get accepted.
Other schools, such as private institutions, often offer a MS only contingent on entry into a PhD program.
Call schools and ask if they offer an MS in bio/physio/anatomy etc... My personal experience has been that a MS has been my BIGGEST selling feature and can only help get you into the "ranking/subgroup."
With average grades and average MCATS a well-done MS may get you that interview at a top 25 school. But an average MS will only reiterate your average performance and could hurt.
So word of advice, take your time, do VERY well, and you will reap the rewards of a graduate education.