I just got a formal letter from one program (the only official contact I've had), but then, I'm still interviewing. I think it's really in late January and February that the bulk of contacts take place, so I'm not worrying yet. Not sure if an e-mail or a letter is preferred....I'd tend to go with the letter, just because I think it looks more official and I like stationery. 🙂
My question is, what do you say to the programs in response to their letters?
ETA: Oops. Asher's post reminded me that I did get a call from another program after they met about my application, but that program said it always calls applicants several weeks after the interview to give them a sense of where they stand. However, most programs that I've visited have told me either that they don't contact applicants, period, or that they call toward the end of the season. I've communicated informally with a couple of interviewers, primarily those with whom I shared a common interest -- if you're really interested in a program, perhaps writing to interviewers who share your interests might be one way of indicating that you've thought about the program and would be a good intellectual fit.