I recently asked a program coordinator whether their faculty prefer emails or snail mail. He said to email the faculty, b/c then the faculty can easily reply if they so choose, and get them closer to the interview while you're fresh in their mind. He also said every email generally gets forwarded to the program coordinator to print off and put in the applicant's chart, so put some time into the email. Might look like poor form if all the emails in your chart read the same, but otherwise email seems OK.
On the other hand, my PD said a hand-written note just to the PD and chair would be totally fine too. Didn't feel it necessary to write a note to each interviewer.
Think you just wanna avoid being the guy who didn't send any thank-you's at all.