I've tried to make it around to visit all of the schools to which I've applied. Based on my experience, I'd shoot for business casual.
This was my approach, flawed as it was:
Suit and Tie to the Law Forum in Chicago - (WAY OVERDRESSED)
Suit and Tie to I.U. for scheduled visit with admissions and professor of communications law - (A little too formal, but OK because of my meeting with the dean of admissions)
Suit and Tie to Kent for their open house - (WAY OVERDRESSED)
Kahki's and a nice sweater with a collared shirt at Case-Western, Valpo, NIU, Toledo, UIUC and Depaul (I definately felt that this attire was more appropriate than wearing a suit.)
That just leaves me with Catholic, George Mason, Michigan State and UT to visit. Michigan State is next week and Catholic is the week after.