ok, this may sound totally cheesy...
but i think you can never go wrong with good manners. if you had a great interview, send a letter. if the program is in your top three, definitely let them know. most places are interviewing 10 for each spot. i don't see how it can hurt you to be polite, or even enthusiastic if it's someplace you'd like to train.
btw, i'm not blowing smoke. i sent notes to everyone who interviewed me. not brown-nosing "it was an honor to be in your presence" crap, just "i enjoyed meeting you, thank you for taking the time.." like my momma taught me.
cheers.
firegirl