The withdraw letter should read something like this:
[Your Name] has chosen his/her schools for the 2009 entering class. I wish to express my appreciation for your interest in me and your willingness to be invite/consider me for an interview. I regret that I was unable to give you additional consideration and feel that you should not interpret this as any indication of your suitability for a candidate like me. It is more a reflection of the number of offers I have received this year.
If you have acceptances from other capable candidates, I wish you the very best of luck and hope you will think of me when you consider your graduate medical education candidate choices. If I can be of any assistance in your future planning, do not hesitate to contact me. However, due to the number of schools who wish to enroll me, I will not be give you a review of your school.
Sincerely yours,
[Your Name], future MD