This question might be better posed in a specific school's thread.
There was a writing portion on my interview, which the "Interview Feedback" survey available on SDN clued me into. If you suspect, bring a pencil with an eraser. (They had golf pencils...)
Interviewees had their choice between 2 very open-ended questions. The essay was 100 words - nothing, in terms of length. It was expected to be completed between activities (and, really, there was plenty of time).
I was distracted by nervous energy & talking with the other applicants, so I found it a little difficult to concentrate on the task at hand, but I got it done. I think I rambled - it certainly wasn't my best work. I think as long as you are able to express thought on paper fluently, you'll be fine.