I emailed a thank-you letter to an interviewer the other day. It's only the second time in my career that I've done this. Generally, I think that thank-you letters look more professional on paper. It's too easy to click "send," and maybe that's my point - that subsequently, not as much effort is put into the letter itself. Or maybe it's the potential for appearing as though not as much effort is put into an emailed thank-you. It's like sending evite invitations to a wedding - a little too casual for the occasion.
I will concede that emailed thank-yous reach an interviewer much faster. So if you decide to go this route, be sure that your email is formatted to look like a formal letter: include the date, title of the interviewer, and the company's name and address. It's a seemingly small detail, but it looks more professional.