Here's what I would be tempted to write, if I were in your very enviable situation:
Dear Institution,
The 2007-2008 application year resulted in my interviewing at many excellent medical schools. Unfortunately, there will be a limited number of institutions that I can attend. Based on a review of your credentials, I have determined that you will not be granted my enrollment. This decision indicates the highly competitive nature of the admissions committees who are recruiting me, and does not reflect upon your qualifications to teach medicine successfully.
I appreciate your interest in my application and hope your class is ultimately filled.
Cordially,
Cranial Gavage
(Don't actually send this; you may apply there for a residency!)