I sent thank you letters to everyone, chairmen and program directors, and have emailed residents up the wazoo - so I doubt that programs are giving me the cold shoulder because they feel slighted
I'm not sweating the lack of thank you letters - from reading past SSN posts, they really don't seem to mean anything, and have no real correlation to successful matching. If anything, some people are misled by them, and assume that a letter of interest means a promise of a match spot. At the Brigham interview, they said flat out that they *never* contact applicants after the fact, because they do not want to mislead folks.
Also, no second looks for me. I left for Bangladesh immediately after my interviews were completed, and will be here doing a public health project until around Rank List submission time - so 2nd looks would be impossible.
I think that if you have received a personalized letter from a program, you should feel good that you made a very favorable impression, and that you are a strong candidate - but obviously don't bet the house that a program is either interested/disinterested, based upon the presence/absence of follow-up contact