You will need a current MCAT score on your AMCAS application in order to apply to CMS. So if your 21 was absent from your BMS application, you will need to add it in order to be considered for CMS. Some people forgot about this last year, but you must satisfy ALL requirements for CMS (not just BMS) in order to get your interview and get in. If your MCAT score is current, you'll probably be fine with a 21 so long as you get As and Bs in BMS (but there is sadly no guarantee).
Just so everyone knows, there were quite a few rejections over the summer for students who got Cs in BMS last year. So remember, it's not about just getting a 3.0; its about not getting Cs at all, and getting as many As as possible!
See you all in a few short weeks!