I wrote such a letter to the associate dean of admissions at my top choice school, after my interviewer recommended I do so (he said he recommends all his interviewees do this). I sent the letter a couple weeks before the committee met to review regular decision applicants, at the end of January. I received my acceptance letter shortly after that. Whether or not the letter helped,I'm not sure, but I'm sure it didn't hurt. Just make sure you are sincere if you send such a letter. I've seen some people mention on SDN that med schools communicate back and forth about applicants later in the cycle, so if you send a letter to more than one school it's possible they might find out.
Anyway...in case this might be of any help, in my letter I stated the reasons that school is my top choice, not in a bulleted list (as some have recommended on this board) but in the body of the letter (paragraph). I also thanked the dean for taking the time to talk with the interviewees on the interview date, and mentioned that I noticed how enthusiastic their students seemed about the program.
Hope this helps...Good luck! 🙂