i think that, unless the admissions office wants everything to go through them, email thank yous are fine if you even send them at all. it's too time consuming to send letters via mail, not to mention expensive. cuz when you think about it you return home the night of your interview at the earliest. thank you note goes out the next day, sorted and processed that day or the next, shipped to wherever for 2 days or so, then processed and delivered. so it's about a 5 day ordeal minimum. that's long after the interviewer has written up their little report and might actually be after the admissions committee has met.
bottom line: don't worry about thank you notes too much, and email notes are fine imho.