Most people would agree that a thank you note to each interviewer would be acceptable. However, I think that is overkill. I think it should be sufficient to send a mailed thank you card to (a) the program coordinator thanking him for his time / setting up the interview day / making sure you got from site to site / etc; (b) program director; and (c) one interviewer with whom you had particularly meaningful contact (eg., talked about a research project, shared interest, offered to introduce you to another attending at another program, etc).
It all goes in the same file anyway.
Sending the note within the week after the interview is appropriate.
If you didn't have any meaningful contacts, then you should leave out (c).
If you actually want a reply from (c) and have substantive follow up questions, then an email would be appropriate. You are probably more likely to get a response to an email than to a thank you card.
Broomdy: Why would you think now is an appropriate or reasonable time to begin professing your love for a program that was probably one of your first interviews and to which you probably (ie., this early in the season) have few or no programs to compare? Go to the rest of your interviews. Check out the other programs. Make an informed decision. When you actually have a #1, then it is an appropriate time to tell a program that they are your #1. How is this not self evident, and why would you need someone on a semi-anonymous Internet forum to give you this advice?
-AT.