I don't know if things have changed since I graduated several years ago, but when I was at CMU my biology class was a great group of students. Very smart, very qualified for medical, dental, even law (biotech type stuff). One of my classmates was a Rhodes scholar. I think it might be a little unfair to say that CMU isn't a 'strong' biology school- I think it might get a bad rap because the department is so small. The first year has a lot of large lectures, but soon enough people 'weed' themselves out of biology, and you end up with a much smaller class in sophmore, junior years. As a result we got a TON of personal, individualized attention! One of my classes had only 9 students in my junior year. So, it ended up being pretty easy to get great letters of rec because we got to know our professors! Friends from other schools tell me they barely spoke a word to their profs because there were so many bio majors.
I don't know the exact stats on admittance rate... I think that it might be high possibly because the biology class is so small... but I will say that I did the pre-med route and went off to ivy league universities after CMU. I'm still studying at Harvard for a post-doc degree. I recently randomly met a girl who graduated 2 years ago who is now at Harvard School of Dental Medicine.
In the end- looking back now, I think going to CMU probably made me look 'unique' and helped me. Schools know that CMU is challenging, and also they don't get as many applicants from CMU. Hope this helps.