I went to two Interview Days last year, and at both about 90% of applicants were wearing suits. (Incidentally, 8% were wearing nice business-casual, and one guy was in jeans). Suits are the norm, and are what is expected. If you have a suit, or can reasonably afford one, wear it to the interview. If not, going with nice business casual (i.e., basically a suit sans jacket) shouldn't hurt your chances.
Some more thoughts... regardless of whether these suit numbers are true across the board, doesn't mean you have to follow. (Unless you're much more comfortable as a follower). There has been some migration to gray/black suits for these interviews, and the faculty I've talked to think it's unnecessary. I also do not agree that it's "expected". To be honest, it doesn't matter
that much! Of course don't wear jeans to your interview (duh) - but slacks/skirt, a nice collared shirt or turtle neck, a blazer -- this will NOT in any way hurt your chances. Do you really think the POI is going to choose someone based on suit versus no suit? Absolutely not! Do you already have a suit that you like and want to wear, then go for it!
There's been talk of khakis as well. If you don't wear khakis in your real life, I would not recommend them for interview weekend. We want to see you! (yes, maybe not stained t-shirt, torn jeans you) but YOU. Dark denim can look just as nice as khakis if paired with a nice shirt. This is not a corporate interview.
I'm a 5th year clinical phd student - which means I've been through my own set of interviews, plus 4 years on the other side. I also just interviewed for internships, and did not wear a suit. I looked very professional, and also felt like myself. A conservative dress, nice skirt or slacks, blazers... all good options that don't require a $200 suit.
So look professional, try to add a little personality, and don't over think it!