Bryn Mawr = very selective. You probably need at least a 3.5 gpa to be competitive, and good SAT or GRE scores. Not sure how many apps they get, but I've heard rumors of about 200+ applicants for the 75 slots. The good things about them is that, once you are in, your odds of completing the program and getting a spot in med school are very hight. The attrition rate is super low (maybe 5 people per year, out of the starting class of 75), and their acceptance rate to med school for those who finish is close to 100%. I was recently accepted into their program, so you can PM me if you have questions about the app process.
If you like Bryn Mawr, you might want to check out Goucher, which I was also recently accepted into. Similar program, but smaller (only like 30 people).
UPenn = moderately selective, from what I've heard. If you have a 3.0 or better, you should stand a good chance of getting in. (Not as familiar w/ their program, though, so I'm not 100% sure about this one).
Harvard = easy admission. Plus, even if you don't get into the program, the courses are open enrollment, so you could still do the curriculum on your own.
The disadvantage of the later two is that the attrition rate is much higher. At BM and Goucher, you'll have a tough time getting in, but once accepted they will really work with you to make sure you get through the program and get into med school. At Penn and Harvard, you won't have as tough a time gaining entry, but once in you will be competing against a lot of other people for grades, attention from the advisers, etc.
Harvard and Penn are still great programs, though, so if they are the right fit for you, you can't go wrong with them.
Good luck.