Here is my letter. Feel free to use it.
Dear Admissions Committee:
After careful review of your credentials, I regret to inform you that University of xxx School of Medicine will receive no further consideration for my attendance for Fall 2005.
This year, I faced a most difficult task in deciding which of the many highly qualified acceptances is best suited for the next step of my professional career. It is evident, indeed, that many competent schools will be unable to gain my attendence to their institutions this year.
Although you may be disappointed, my personal wish for you is that you find other "qualified" applicants to fill your 2005 entering class. Thank you for your patience throughout this process and PLEASE REFUND my deposit as soon as possible. Thank you.
Sincerely,
Sdnstud