I would always dress "professionally" because I do not know what the guy on the other side is going to think. I don't want the start of the interview to be the end of it. When I talk to the interviewer I can know if that person is someone I can be working with, I always ask what the dress code is like and I assure you it makes no difference.

One company was 100% casual (don't wear a miniskirt and you are fine). The company was the biggest POS to work for.

Another company is business casual (occasionally they don't mind me wearing a t-shirt and jeans) and it is is just a pleasure to work at this one.

Edit: I actually like wearing business casual, very comfortable cloths.