hey quick question guys, did you guys send a thank you letter to your interviewers by mail or e-mail? thx.
oh ya, if it was sent by mail, did you send an actual letter or a card?
Unless you ask a question that requires a response, you can't expect an acknowledgement of a thank you email.
This year, I've received expressions of thanks from about 1/3 of the applicants I've interviewed. It has been 50-50 email and snail mail. Most people who send something by mail use a little fold in half card commonly sold in packets of 6 to 10.
I would say it largely depends on the schools' manner in which they gave you the interviewer's contact information. At most schools, I was given a physical address and no email--which would imply that anything I need to send them would be sent via post. At the same time, one school gave me a list of emails, so I chose to write emails. When given both, I sent it by hand, because a hand-written card (IMO) is a bit more meaningful. Keep in mind this opinion is coming from someone with horrid handwriting!
Me: "Do you think I should send a thank-you note to my interviewers?"
Him: "Oh god no. It makes me gag when people send those to me after an interview."
Him = my advisor, a physician/scientist at Mayo
EDIT: Not to say you should never write a thank-you note, but just keep in mind that not everyone likes receiving them. Just be careful with your words should you choose to do so.