My mom informed me that I received a letter in the mail from Columbia. It basically said something along the lines of "I appreciate you sending a thank-you note to [guy I interviewed with]. We send out acceptance/rejection notifications in February, so don't worry if you don't hear from us by then. We are very strongly keeping you in mind." It was personally signed by the Franz guy. So the Thank You note did have some effect, at least enough to have Franz mention it in this letter. For my U.Mich interview, I did not send any of the three interviewers a Thank You note... and I received that e-mail posted in another thread. I guess the take home message would be when it doubt, send a TY note.