I've interviewed at two locations for clinical psychology, and every candidate was formal, ie- business suits, nice shoes, portfolio bags, button down shirts etc. During the reception, business casual was an unwritten rule. They never told us about a dress code. The one person who was more casual (wearing a nice button down, clean well-fitting jeans and dress shoes), stood out, and everyone talked about him the entire weekend and how inappropriate he was. By everyone I mean faculty, staff, grad students and other candidates.
At the same time, when I was just with the graduate students, it was really about getting to know each other and making sure we can work together and spend time together, not just as colleagues, but as friends. They really looked down at candidates that were not social, both during the official dinner reception, and when we went out to dinner on our own casually.
